Williams began preparation for the first grand slam of the year at the Hopman Cup, where she partnered compatriot John Isner to represent team USA. The pair played their first tie against Italy. Williams got off to a slow start in her singles match against Flavia Pennetta, unable to win a game in the first set. However, a mid-match coffee helped the American turn things around and win the match in three sets. She then teamed up with Isner to win the mixed doubles in a super tiebreak. In the second tie of the exhibition, Williams faced off against Canadian youngster Eugenie Bouchard. The American suffered her first loss in her Hopman Cup history, having previously gone undefeated in her last two campaigns, managing to win only three games. Later Williams teamed up with Isner to win their mixed doubles match in straight sets. In the final round-robin tie of the tournament, Williams came up against Czech lefty Lucie Šafářová in an encounter that lasted over two and a half hours, with the American narrowly clinching victory in a third set tiebreak, 8 points to 6. Team USA also won their mixed doubles match, putting Serena in the Hopman Cup final for the third time in as many attempts. In the final Williams met Poland's Agnieszka Radwańska, to whom she had never lost before. The world No. 5 was in inspired form and took the match in three sets. Williams' teammate Isner managed to salvage and send the tie in to a live mixed doubles rubber after winning his singles match against Janowicz, however, the Poles proved too strong for the Americans and took the Hopman Cup with a two set victory.
At the first major of 2015 Williams opened her campaign for a 6th Australian Open title against Alison Van Uytvanck of Belgium. The world No. 1 was dominant in taking the first set without dropping a game in 21 minutes. A tighter second set followed giving the American her 15th first round win at the tournament in as many tries. In the second round Williams came up against former world No. 2 Vera Zvonareva, a rematch of the 2010 Wimbledon final. Williams fell behind and then responded by rattling off ten consecutive games to win in two sets and advance to the final 32. Serena was also scheduled to play doubles with her sister Venus, drawing Anabel Medina Garrigues and Yaroslava Shvedova in the first round, however the pair pulled out without giving an official reason before the doubles tournament began and were replaced by an alternate team. In the third round Williams came up against Ukrainian counter-puncher Elina Svitolina and after overcoming a slow start, which saw her drop the first set, she fought back to take the match in three sets, hitting 45 winners to her opponent's 27. The trend of sluggish starts continued in the fourth round where Williams came up against her conqueror from the previous year's French Open, Garbiñe Muguruza. As in their previous meeting the young Spaniard took the opening set from a flat-footed Williams. The pair exchanged breaks early in the second set but the world No. 1 raised her level to earn the decisive break in the eighth game and take set two. After a difficult opening service game in the third set, in which Williams saved six break points, the American blasted a total of 15 winners to take the third set and the match.
At the quarterfinal stage Williams earned her most comfortable victory of the tournament against the previous year's surprise finalist Dominika Cibulková. The top seed hit 15 aces and dropped just four games en route to setting up a final four meeting with her sister's conqueror Madison Keys. Williams had never failed to win the title in Melbourne once making it to the semifinal stage, however, she was broken in the first game and struggled to get in to her Fed Cup teammate's service games. Williams eventually broke back in sixth game and won the set in a tiebreak. The world No. 1 coasted in set two and closed out the match on her ninth match point. With passage in to her 23rd Grand Slam final Williams secured she would maintain the number 1 ranking. In the final she came up against her long-time rival and world No. 2 Maria Sharapova to whom she had not lost to in 15 consecutive meetings. Williams began the match aggressively and broke Sharapova's serve immediately. The match continued with serve until the pair traded breaks in the seventh and eighth games, then the world No. 1 broke to love to take the first set from the 2008 champion. The second set was more tightly contested with both women creating break point opportunities but ultimately going with serve to a tiebreaker. Sharapova saved two championship points on her own serve, but Williams' won on her third championship point for her 19th major title. Williams ended the match with 38 winners, including 18 aces and only 25 unforced errors to Sharapova's 21 winners, including 5 aces and 15 unforced errors. With this win Williams broke her tie with Chris Evert and Martina Navratilova for second most Grand Slam singles titles in the Open Era and put herself only three major wins behind Steffi Graf's 22.
Fresh off her victory in Melbourne, Williams flew to Buenos Aires to represent the USA in their tie against Argentina in the Fed Cup World Group II. Serena was nominated to play her first rubber against Argentine No. 2 Maria Irigoyen. Williams initially struggled to adapt to the clay as the pair went with serve for most of the first set. Williams then won the final eight games of the match to win in straight sets. Williams was nominated to play her second singles rubber against Argentine No.1 Paula Ormaechea, however she was replaced by Coco Vandeweghe due to illness. USA won the tie 4–1 and advanced to the Fed Cup World Group Play-offs.
Shortly after winning the first Grand Slam of the season, Williams, in an exclusive column for TIME magazine, announced her intention to end her 13-year boycott of Indian Wells and return to the tournament for the first since winning the title in 2001. Her highly anticipated opening match was set for 13 March when Monica Niculescu advanced to the second round. Williams' return to Stadium 1 at the Indian Wells Tennis Garden was welcomed with rapturous applause and a standing ovation from those in attendance, including tournament owner Larry Ellison, co-founder of Microsoft Bill Gates and tennis legend John McEnroe. Despite the warm reception Williams was tense and was broken immediately by Niculescu, who went on to serve for the opening set in the tenth game, but Williams' resilience saw her win three consecutive games to take the set. In the second set, the pair traded breaks early on and, just as in the first set, the American got the decisive break in the twelfth game to win the match. In the third round Williams made quick work of an error-strewn Zarina Diyas, losing only two games, concluding with a second set bagel. In the round of 16, Williams faced compatriot Sloane Stephens. A sharp and engaged Stephens got an early lead, taking the first three games, but Williams responded by winning the next three to level the match. The set remained on serve with Stephens saving a set point in the tenth game to force it to a tiebreak, which the younger American won. Williams fought back breaking Stephens' serve a collective four times over the final two sets to advance to the final eight. At the quarterfinal stage Williams took on an in-form Timea Bacsinszky whom advanced to the final eight off the back of winning back-to-back titles in Mexico. The Swiss led the opening set in its early stages, however, as in her opening match, Serena came from behind and claimed the first set, with the decisive break coming in the twelfth game. The second set started with four consecutive breaks of serve. Williams righted the ship with a break of serve in the eighth game, which she backed up by serving out the match to 15 to secure a spot in the semifinals. Williams finished the match with 23 winners to her opponents' 11, including 7 aces. Williams was set to face world No. 3 Simona Halep at the semifinal stage but had to withdraw before the match due to a right knee injury. After her withdrawal was announced to those in attendance Williams came out on to Stadium 1 court to address the crowd, apologize for her withdrawal and reassure fans that she would be back in 2016.
Williams returned to Miami the following week with aims to defend her title and to further extend her record and become the only player to have won the event eight times. After receiving a bye in to the second round as the top seed, Williams was pitted against Monica Niculescu in her opening match for the second straight tournament. The world No. 1 made light work of the Romanian this time, dropping just four games and showing no signs of the knee injury that forced her to withdraw from Indian Wells. In the third round Williams needed only 41 minutes to overwhelm the incumbent ITF junior world No. 1 Catherine Bellis, dropping just 14 points in her victory over the teenager. Serena then took on old rival and 2006 Miami champion Svetlana Kuznetsova in the fourth round. Williams went untroubled on serve, saving all four break points against her and broke the Russian's serve three times en route to a comfortable victory, the most lopsided win for Serena since 2004. In the quarterfinals the world No. 1 faced off against her conqueror at 2013 Wimbledon Championships, Sabine Lisicki. The in-form German tested the world No. 1 with her big return game but despite leading Williams by a break twice in the first set she was broken back immediately on both occasions. Williams eventually took the first set in a tiebreak and broke Lisicki's serve to begin the second set, however Williams began to struggle with the sun while serving and went on to commit 17 unforced errors to drop the set. Frustrated with her play, Williams refocused and cleaned up her errors to gain a decisive break in the second game and take the set and match in the third. This victory marked Williams' 700th career win, the most among active players and ninth on the all-time list. In the semifinals she faced third seed and the last player to beat her, Simona Halep, in a match that was originally set to take place two weeks earlier. Williams came out firing, blasting 13 winners to Halep's 6 to take the opening set. The second set went with serve until errors started to flow from Williams' racket which the Romanian was the beneficiary of in the ninth game. Halep served out the set. The world No.1 ended the set with 20 unforced errors to Simona's 10. Williams struck back immediately breaking Halep at the first opportunity in the third set. Williams won a tight third set. In the final Williams faced her sister's conqueror Carla Suárez Navarro. The Spaniard had lost all eight sets contested in four previous meetings with Williams and this was no different with Williams winning in 56 minutes. Williams struck 27 winners, including 7 aces, to 16 unforced errors to successfully defend her title and win Miami for a record eighth time. This was also the second time Williams had won three consecutive Miami titles after originally winning the event three consecutive times between 2002 and 2004.
In a quest to gain eligibility for the 2016 Summer Olympics Williams began her clay season in Brindisi in a Fed Cup World Group Play-off tie against Italy. The American was nominated to play Camila Giorgi in her first rubber and edged out the first set in a tiebreak. Williams broke serve for the first time in the fourth game and wrapped up the match with a second break as the Italian was serving to stay in the match. The tie was locked at 1–1 at the end of day 1 after Williams' teammate Lauren Davis managed to win only three games in her rubber. Williams played Italian No. 1 Sara Errani in her second single rubber and lost just her second set in eight meetings against the Italian. The American's fluctuating level saw her go from a break up to a break down with the Italian serving for the match. The American broke serve and won the set in a tiebreak to level the match. Williams took the third set and ended the match with 70 winners to Errani's 7. With this win team USA led 2–1 and needed one more point to advance back in to the World Group. The tie was forced in to a deciding doubles rubber when Christina McHale managed just two games in her loss to Brindisi-born Flavia Pennetta. Williams teamed up with Alison Riske, marking the first time she had played doubles with anyone other than her sister Venus in 13 years, to take on Eranni and Pennetta. The Italian pairing handed Williams her first Fed Cup loss en route to winning the final rubber and clinching the tie 3–2. USA were relegated to World Group II.
After winning a couple of shaky matches on clay in Brindisi, Williams arrived in Madrid to play her first WTA event of the clay season. Her first round opponent was one of the breakout stars of 2015, Madison Brengle. It took Williams just 55 minutes to beat her fellow American, with the loss of just one game, hitting 36 winners to just 18 unforced errors. Williams faced Sloane Stephens, for the second time this season, in the second round. A break of serve in the opening game was enough for Williams to secure the opening set. Stephens failed to win a game in the second set and the No. 1 seed advanced in to the third round in 58 minutes. Williams' third round opponent was familiar foe Victoria Azarenka whom she had not faced in over a year. Despite being down 5–1 in the first set tiebreak, Williams won six consecutive points to claim the set. Azarenka fought back and won the second set. The deciding set went back and forth until William won the tiebreak to advance to the quarterfinals. At the quarterfinal stage Williams dominated clay court specialist Carla Suárez Navarro, hitting 43 winners to 27 unforced errors, for the loss of just four games. This win set up a semifinal encounter with world No. 4 and reigning Wimbledon champion, Petra Kvitová. Williams lost the match in straight sets and was handed her first loss of the year.
With her win streak snapped, Williams returned to Rome as the two-time defending champion. She opened her defense against world No. 39 Anastasia Pavlyuchenkova and won comprehensively in just over an hour, striking 25 winners to only 17 unforced errors. Williams was set to face countrywoman Christina McHale in the third round but withdrew before the match citing a right elbow injury. This was the first time Williams headed in to the French Open without winning a warm-up clay tournament since 2010.
To kick off her French Open campaign, Williams faced off against Czech qualifier and accomplished doubles player Andrea Hlaváčková. The world No. 1 advanced with ease in just under an hour, blasting 25 winners, untroubled by the elbow injury that had forced her to withdraw from Rome. In the second round Williams was pitted against another qualifier, Germany's Anna-Lena Friedsam. The qualifier failed to serve the first set out, but Williams' sloppy play saw her drop the opener. Williams fought back to force a decider, in which she broke in the first and final of Friedsam's service games to advance to the final 32. Her next opponent was close rival and friend Victoria Azarenka. Williams came in to this match having never lost to the Belarusian at a major in eight previous meetings. Azarenka started sharp on return, breaking Williams' serve thrice to lead the world No. 1 by a set and a break. Williams battled to break back in the eighth game to level the set. In the tenth game the Belarusian saved a set point but the chair umpire called for the point to be replayed. Williams took her chance and sealed set. Azarenka, furious with the call, let out her frustration by breaking Serena in her opening service game of the deciding set. Williams responded by winning six straight games to take the match in just over two hours, finishing the match with 41 winners to 21 unforced errors. With this win Williams became the first woman in the Open Era to win 50 matches at each Grand Slam. In the fourth round, Williams faced compatriot Sloane Stephens. Stephens took advantage of a slow starting Williams taking the first set losing just a game and breaking the world No. 1's serve thrice. In the second set, Williams came back to break in the seventh game just to be broken back by Stephens. However, Williams broke again in the eleventh and closed it out to push it to a decider. In the final set, Williams broke the younger Americans serve twice, including in the ninth and final game.
In the last eight, Williams took on Italy's Sara Errani, Williams dominated the match losing only four games in the match including a breadstick in the first set. Williams hit 39 winners including 10 aces. In the final four, Williams faced first time slam semifinalist Swiss Timea Bacsinszky. In the first, Bacsinszky broke Williams in the fifth game and that was enough for the Swiss to serve it out in the tenth game to take the first set. In the second set, Bacsinszky broke Williams again in the fifth game, but Williams came back and won the next ten games of the match to advance to the finals. In the final, Williams faced first time finalist Lucie Šafářová, Williams broke Šafářová in the fourth game and closed it out in the ninth game. Williams took an early lead, leading with two breaks, however as Williams tightened up, Šafářová took advantage and brought it back on serve. Williams then broke in the eleventh game and served for the match but Šafářová broke back to push it to a tie-break. Šafářová dominated tie-break. In the final set, Šafářová took the first two games but Williams came back to win the next six game to win the match. The win gave Williams her 20th slam, being the third person to win 20 or more slams and the third person to win each slam at least three times.
Despite the addition of an extra week of grass court events before Wimbledon, the five-time singles champion, did not play a grass court tune-up event coming into this Grand Slam event. In her opening round Williams faced off against Russian qualifier Margarita Gasparyan, whom she struggled with initially, falling a break behind early in the first set, but Williams bounced back to dispatch her opponent in two sets. Her second round match was against talented doubles player Timea Babos, a match that would last only 59 minutes, with Williams losing only five games. In the third round, against Heather Watson and a partisan crowd, Williams was two points from losing with Watson serving for the match. After 135 minutes, Williams prevailed in three sets. She would face her sister Venus Williams in her fourth round encounter — the earliest the sisters had met at the Championships and the earliest the two had met in a Grand Slam since the 2005 US Open — but it was a routine two set victoty for the younger Williams sister.
In the quarterfinals, Williams faced Victoria Azarenka. For the second consecutive major Azarenka won the first set with her accurate return game, but Williams raised her level in the second set and after a multi-deuce fourth game, in which Azarenka held serve, Williams ran away with the match, winning seven consecutive games with her pinpoint serving. The world No. 1 went on to seal the match in three sets, serving 17 aces, her best serving performance of the Championships. In the semifinals Williams faced off against world No. 4 Maria Sharapova. Williams defeated Sharapova for the seventeenth consecutive time for the loss of six games to advance to her eighth Wimbledon final. In the final she faced major final debutant Garbiñe Muguruza, the young Spaniard who had handed Williams her heaviest defeat at a major, just a year before. Though Williams' opening service game was broken, she recovered and won the opening set. Williams raced ahead to a double break lead and would serve for the match twice, but the young Spaniard broke the world No. 1 both times. Although Muguruza got the match back on serve, she would still have to serve to stay in the match. Williams broke the Spaniard's serve to win her sixth Wimbledon title and complete the "Serena Slam 2.0". The final lasted 83 minutes, with Williams serving 12 aces to 8 double faults, hitting 29 winners to 15 unforced errors, and getting in 37 of 68 first serves. With this victory Serena Williams became the oldest female in the Open Era to win a major singles title, passing the former mark of Martina Navratilova. This win was her twenty-first major singles title, putting her one behind Steffi Graf's Open Era record, and three behind the all-time mark set by Margaret Court.
Immediately following her triumph at Wimbledon, Williams returned to Båstad after missing the previous edition of the tournament because of a viral illness. The world No. 1 faced Ysaline Bonaventure and needed just 53 minutes to collect a win over the Belgian for the loss of three games. Williams was set to play Klára Koukalová in the round of 16, but withdrew due to an elbow injury.
Williams began her US Open Series in Toronto after withdrawing from Stanford with the same elbow injury sustained earlier in the year. After receiving a bye in the first round as the top seed, Williams faced Italian veteran Flavia Pennetta and won in three sets coming from a set and a break down to win. She then beat Andrea Petkovic and Roberta Vinci in straight sets to advance to the semifinals. She lost to 18-year old Swiss Belinda Bencic in three tight sets. During the match she served at 50 percent, committed 59 unforced errors (more than half the points Bencic won) and converted on only 6 of her 20 break point opportunities. It was her second defeat of the year and first on hard court since the 2014 WTA Finals to Simona Halep. The loss also ended William's 18 match winning streak in three-set matches. Coincidentally, the last time she suffered a three-set loss was also in the semifinals of the Canadian Open (against her sister Venus) in 2014.
Williams was the top seed and, after a bye, she defeated Tsvetana Pironkova and Karin Knapp both in straight sets. In the quarterfinals she defeated Ana Ivanovic in three sets and Elina Svitolina in the semifinals in straight sets. She won her fifth title of the year with a two set victory over Simona Halep.
Williams was the top seed and three-time defending champion and hoping to win a calendar year Grand Slam. She started off with Russian Vitalia Diatchenko in the first round where quickly won on the first set, however, Diatchenko retired due to an injury to advance the next round. In the second round, she faced with Dutch Kiki Bertens that lasted in 1 hour and 32 minutes. In the third round, she faced fellow American Bethanie Mattek-Sands where she had a tough momentum that lost on the first set. During the second set, she narrowly avoided upset by Mattek-Sands and quickly regained focus on the third set. In the round of 16, she faced with another fellow American Madison Keys and winning in straight sets. In the quarterfinal stage, she faced her third American opponent in a row, her sister Venus, Serena dominated early in the first set. During the second set, she lost to Venus, and in the third set, she narrowly defeated her sister to advance the semifinals. In the semifinals, she faced with Italy's Roberta Vinci, as the latter was on her first Slam singles semifinal. The semifinal match was supposed to play the night before because of rain delay. Despite with odds, Vinci never beaten Williams in all four previous meetings. She did very well strong at the first set. However, she was upset by Vinci and lost in next two straight sets, ending her bid to win the calendar year Grand Slam since Steffi Graf did so in 1988. It was also on her first Slam loss since the previous year's Wimbledon.
On October 2, Williams announced that she is withdrawn in both China Open and the WTA Finals due to injuries. Williams is a defending quarterfinalist in the China Open, while she is a three-time defending champion in the WTA Finals that she is considering to be a fan favorite.
Williams' 2015 singles tournament schedule is as follows:
Williams' 2015 doubles tournament schedule is as follows:
Ordered by percentage of wins
Figures in United States dollars (USD) unless noted.