Williams had won 12 successive matches in ranking events after winning the World Championship and LG Cup titles, but admitted he had been “outplayed” as Stevens beat him 5-1 in Brighton.
“Matthew played some really solid stuff and I didn’t get much of a chance to get into the match,” he said.
“He outplayed me and I fancy him to go on and win the tournament now.
“It’s nice to see him back playing some good stuff. He’s proved you don’t have to play frame-winning breaks all the time, you just have to be solid, play good safety and take your chances.”
Ronnie O’Sullivan survived a thrilling battle before finally booking his place in the semi-finals.
The ‘Rocket’ came close to elimination at the hands of giant-killer Gerard Greene but won the last two frames to record a hard-fought 5-4 victory. However, O’Sullivan only went through after Greene, the world number 38, suffered a huge slice of bad fortune in the deciding frame.
Greene, who sent Ken Doherty and Joe Swail packing earlier in the week, had a chance to clinch victory but in potting the pink from the last red, the cue ball cannoned the black and found a middle pocket.
It presented O’Sullivan with the perfect opportunity to clinch the match on the colours, and he took it to scrape into the semi-finals.
O’Sullivan said: “You don’t like to see something like that happen in a decider. It’s a nightmare but it’s happened to me before and you have to deal with it.
“I’d have rather have gone through by potting a good ball. It’s not an ideal way to win but sometimes the balls have a mind of their own.”
O’Sullivan will face Stephen Lee for a place in the final after the world number five beat defending champion Paul Hunter 5-3.