Stevenson made her professional tennis debut in 1999 at Wimbledon, two weeks after graduating from La Jolla Country Day School. Stevenson became the first woman qualifier in the tennis open era to reach the semi-finals. It was her second time at Wimbledon, having competed the year before in the Junior Wimbledon Championships. In 1999, Stevenson qualified to the quarterfinals at Birmingham, a grass warmup to Wimbledon, before withdrawing to Magüi Serna because of a stomach muscle pull. The next week, Stevenson was the number one seed during qualifying at Roehampton—and did not drop a set in three rounds as she moved into the main draw at Wimbledon. Stevenson beat number eleven seed Julie Halard in the third round. In the fourth round, she saved one match point against Lisa Raymond in a 2–6, 7–6, 6–1 win. She then beat Jelena Dokić in three sets in the quarter-finals, before losing to the eventual champion, third seed Lindsay Davenport. Stevenson had 57 aces during the fortnight, her serve speed over 120 mph. It was the beginning of Stevenson being known for the fastest second serve in the women's game. Stevenson's second serve was 105 mph – 115 mph. Phil Knight, the co-founder and chairman of Nike, flew to London to personally sign Stevenson to a three-year contract.
During the months following her dramatic run at Wimbledon, Stevenson was named rookie of the year by Tennis Magazine and named Most Fascinating by People Magazine. She was interviewed by Barbara Walters for a Barbara Walters Special and was featured in a variety of national and international media. Nike flew a dozen flags with Stevenson's name in bold print during her first visit to the iconic campus. Stevenson moved from California to train with Nick Bollettieri at IMG Academy from 1999–2002. In 2000 and 2001, Stevenson, often pitted against top 20 players – including Nathalie Tauziat, Mary Pierce, Martina Hingis, Monica Seles, Amanda Coetzer, Lisa Raymond, Dominique Van Roost, Julie Halard, Jennifer Capriati, Serena Williams, and Venus Williams – worked on her aggressive all-court playing style as she found her way in the professional game.
Stevenson reached the quarterfinals at Quebec City in 2001 and the quarterfinals at Linz, defeating Arantxa Sánchez Vicario en route.
But, it was in 2002 that Stevenson moved up to No. 18 in the world. Stevenson had played during the European indoor season in 2001 and was the only American player to appear in Moscow in 2001 following the September 11 terrorist attacks. In 2002, prior to the European indoors, she lost to Monica Seles in the Tokyo quarterfinals 7–6(11–9), 7–6(11–9), and there were no breaks of serve until the tiebreakers. Alan Mills, the Tokyo referee and renowned Wimbledon referee proclaimed, "This is the finest women's serving match I have ever seen." Soon after, Seles and Stevenson were asked to serve aces for the American Heart Association, bringing awareness to heart disease. Stevenson served 100 aces in 2003. During the 2002 European indoors, Stevenson won her first doubles title with Serena Williams in Leipzig, Germany. She reached the quarterfinals of Filderstadt, defeating number one Jennifer Capriati. She was a finalist at Linz, beating two top ten players en route, including Capriati. Stevenson finished the year at 18, percentage points away from No. 17 Elena Dementieva.
Stevenson experienced shoulder pain in 2003 at the Wimbledon Championships. She worked on strengthening the area, but was unable to repair the tear. Stevenson went to Birmingham, Alabama, where Dr. James Andrews performed a Type II labral repair on her right shoulder in September 2004. In 2006, following 18 months of shoulder strengthening, Stevenson began to mount a comeback.
Stevenson played in the qualifying round at Wimbledon 2006, but after defeating Jelena Dokić, a player that she had played in 1999 in the Wimbledon quarters, when they were both qualifiers, she suffered a pectoral strain and fell in the second round of qualifiers. At the 2006 Cincinnati Women's Open, she also had a successful run in the qualifying round, but continued shoulder pain caused her to fall in the final round to Chin-Wei Chen 3–6, 6–3, 7–5. By 2009, Stevenson gained shoulder strength and posted strong results. In 2010, Kevin Wilk, Dr. Andrews physical therapist said that Stevenson's shoulder "feels like a non-surgical arm."
Stevenson won the Sarasota Clay Court Invitational in April 2012.
Stevenson joined Venus Williams, Serena Williams, Lindsay Davenport, Monica Seles, Mary Pierce and Jennifer Capriati as a premier power player on the WTA Tour in 1999. 6' 1" and right-handed, she plays with a one-handed backhand. Her serve, forehand, and one-handed backhand are noted weapons in Stevenson's all-court game. Her fastest serve was clocked at 125 mph. She had the fastest second serve in the women's game from 1999–2004 at 105–115 mph. She was the first woman to amass 57 aces during the Wimbledon fortnight in 1999. The power game came from years of repetitive lessons. At nine years old, Stevenson began traveling from her home in San Diego to Los Angeles to be coached by Robert Lansdorp and Pete Fischer. It was Lansdorp who developed her powerful ground game, changing her two-handed backhand to a one-handed backhand. Lansdorp would tie her arm with an ace bandage to work on the backhand motion. Fischer, who also coached Pete Sampras, developed Stevenson's service motion, often used by coaches to teach "the perfect service motion." Fischer designed service drills to resemble Sampras' fluid serve.
Stevenson's mother is Samantha Stevenson, a noted sports journalist. Her father is former basketball player Julius Erving. Stevenson met her father for the first time in October 2008 after she initiated a meeting. The meeting was documented by ESPN.com's "Reaching Out". Stevenson keeps up with her father between tournaments.
On September 11, 2001, Stevenson lost one of her friends – Manny Del Valle, a fireman in the World Trade Center. Stevenson wrote an article in the New York Times about Del Valle, a driver to the players at the US Open. In 2002, the four Grand Slams allowed Stevenson to wear the patch of DelValle's Engine Company (Engine No. 5) on her Nike hat.
Stevenson graduated from the University of Colorado in December 2007, with a Bachelor of Liberal Arts, Sociology. She graduated on the Dean's List and is the only Final 8 member to have graduated from college while playing professional tennis. Stevenson was inducted into the La Jolla Country Day School Hall of Fame in December 2009 – joining fellow Torrey, Rashaan Salaam, the 1994 Heisman Trophy winner.
Updated June 8, 2008
A = did not participate in the tournament
SR = the ratio of the number of singles tournaments won to the number of those tournaments played
_ = tournament was either not held or not a Tier I event
1 If Fed Cup (0–1 overall) participation is included, her record stands at 213–220 overall