Lewis won the World Masters and WDF World Cup boys' events in 2009.
Lewis reached the last 24 of the 2010 World Masters, losing 3-2 in sets to Ted Hankey. He also reached the semi-finals of the 2011 WDF World Cup, losing out to Martin Adams 6-3.
In January 2012, Lewis successfully earned a Professional Darts Corporation tour card on day four of the PDC Pro Tour Q School. This enabled him to participate in all this and next year's Players Championships, UK Open Qualifiers and European Tour events. Lewis competed in his first UK Open in 2012, defeating Steve Farmer and Mark Stephenson before falling to Pete Hudson in the second round. In August, Lewis beat John Scott and Michael Barnard in the UK Qualifier for the fourth European Tour event of the year, the German Darts Masters, but then lost 3–6 to Richie Burnett in the first round in Stuttgart. He also won twice on the Youth Tour throughout the year.
Lewis finished fourth on the 2012 Youth Tour Order of Merit, but was the highest player who had not already qualified for the 2013 World Championship. In Lewis' first appearance in the tournament he lost to the Philippines Lourence Ilagan in the preliminary round by four legs to three, despite leading 3–1. He was ranked world number 68 after the event. At the UK Open, Lewis lost 5–3 to Tony West in the preliminary round.
Lewis reached his first final on the main tour at the Gibraltar Darts Trophy. He beat Colin Osborne 6–4, Robert Thornton 6–5, Steve Brown 6–0, Kevin Painter 6–5 and two-time world champion Adrian Lewis 6–5. His match against Lewis guaranteed him a spot in the World Matchplay for the first time and Lewis stated that that match was his final. In the final itself he was defeated 6–1 by 16-time world champion Phil Taylor. He played Painter in the first round of the World Matchplay and was defeated 10–7. His best results in the rest of the year were in two Players Championships where he lost in the quarter-finals to Mensur Suljović and Jamie Robinson. His performances over the year saw him finish 32nd on the ProTour Order of Merit to claim the final spot in the Players Championship Finals. In Lewis' debut in the event he faced top seed Michael van Gerwen in the first round and lost 6–1.
His ProTour ranking saw him qualify for the 2014 World Championship by claiming the third of sixteen spots that were available to players outside the top 32 on the main Order of Merit. Lewis played Raymond van Barneveld in the first round and was beaten 3–0, losing each set by three legs to one as he hit only three of his fourteen darts at a double. Lewis had risen 28 places during the year to be ranked world number 40.
In February 2014, Lewis reached the second Pro Tour final of his career by averaging 108.07 in defeating world number one Michael van Gerwen 6–4 in the semi-finals of the fourth UK Open Qualifier, but he could not replicate the performance in the final as he was beaten 6–1 by Brendan Dolan. He advanced to the fifth round of the UK Open for the first time a month later but was beaten 9–5 by Mensur Suljović. During the rest of the campaign, Lewis was beaten in the last 16 of three Players Championships and qualified for three European Tour events, but could not get past the second round.
Lewis had taken out finishes of 141 and 125 to be 1–1 against James Wade in the first round of the 2015 World Championship. He won the first leg of the third set, but then lost six legs in a row to be knocked out 3–1. His first semi-final appearance in over a year came at the Gibraltar Darts Trophy where he was beaten 6–1 by Michael van Gerwen. Lewis was now the number two Welsh player on the Order of Merit behind Mark Webster and they teamed up at the World Cup, but suffered a surprise 5–3 loss to Hong Kong in the first round. He defeated Justin Pipe 10–7 at the World Matchplay, before losing 13–2 to Michael van Gerwen in the second round. He threw the first nine-dart finish of his PDC career at the 17th Players Championship.
At the World Grand Prix, Lewis survived two match darts from William O'Connor to edge past him 2–1 in the first round and then came from 2–0 down in sets against Michael Smith to win 3–2. In his first major event quarter-final he was defeated 3–0 by Van Gerwen. Robert Thornton missed one match dart in the first round of their Players Championship Finals to allow Lewis to win 6–5. He led Dave Chisnall 8–6 in the second round, but lost four legs on the trot, missing three darts to force a deciding leg in the final leg.
Lewis lost 3–1 to Daryl Gurney in the first round of the 2016 World Championship. In the second UK Open Qualifier, Lewis reached the semi-finals and was narrowly beaten 6–5 by Alan Norris. This gave him entry in to the third round of the main event and Stephen Bunting hit legs of 12 and 11 darts to move from 8–7 down to win 9–8. In the penultimate Players Championship event, Lewis eliminated the likes of Benito van de Pas and Robert Thornton to play in the quarter-finals, where he lost 6–5 to Simon Whitlock. He recorded a 6–4 win over Jamie Caven in the opening round of the Players Championship Finals, before being knocked out 6–4 by Dave Chisnall.
Lewis won a match at the World Championship at the fifth time of asking when he edged past Mick McGowan 3–2. He won one leg as he was thrashed 4–0 by Peter Wright in the second round.2013: Preliminary round: (lost to Lourence Ilagan 3–4)
2014: First round (lost to Raymond van Barneveld 0–3)
2015: First round (lost to James Wade 1–3)
2016: First round (lost to Daryl Gurney 1–3)
2016: Second round (lost to Peter Wright 0–4)