PICTURES by Tai Chengzhe
Williams had lost eight consecutive ranking matches in a dozen years against the Rocket and his barren streak looked likely to continue when he trailed 3-0 in Chengdu. But Williams, who was rated a 300/1 outsider by some bookmakers before the tournament started, fought back to win a thrilling contest and will now face Mark Allen in the last four on Saturday.
Two-time World Champion Williams, age 39, is chasing his first ranking title since the 2011 German Masters and is now just two wins away from the £125,000 top prize.
Williams struggled to get going in the early stages and his opponent went 2-0 up with a top break of 65. O'Sullivan then made a 44 clearance which included a stunning pink to extend his lead. Williams pulled one back before the interval then found his rhythm with runs of 120 and 59 for 3-3.
In frame seven, O'Sullivan's missed blue from short range on 27 allowed Williams to edge ahead, and the Welshman kept his momentum going with a smooth 77 for 5-3. Back came O'Sullivan with 72 to win frame nine, and he dominated the next to set up the decider.
Williams had first chance and made 50 before running out of position. O'Sullivan cross-doubled a red to give himself a chance to counter, but then missed a tricky black and Williams doubled a red himself before adding 22 to seal the result.
"I haven't beaten him for so long, eventually I've got rid of that pain tonight," said world number 22 Williams. "I was shocking for the first few frames today and dragged Ronnie down. After the interval I was a different player. I made some good breaks and I'm over the moon to get through.
"It's an excellent win for me, even though Ronnie wasn't at his best. We're both the same age but he can produce a lot better snooker than the likes of me and John Higgins. He's still the best player in the world."
O'Sullivan, who is yet to win a title this season, said: "We both started poorly and I was struggling with my concentration. Then he played well to go 5-3. He's in good touch. At 5-3 I felt I had to make something happen, and that was the best I felt in the match."
Allen also had to come from behind as he overturned a 3-1 deficit to beat Michael White 6-3. White, who had hoped to reach his first ranking semi, top scored with 68 in winning three of the first four, but Ulsterman Allen then reeled off five in a row with top breaks of 84, 76, 58 and 58.
World number eight Allen is targeting his fourth final of the season having won the Paul Hunter Classic and finished runner-up at the Riga Open and Shanghai Masters. He said: "Even at 3-1 down I felt that if I played well I would go on to win, and I did that after the interval. Michael was unlucky because at 3-3 he had a good scoring chance but got a kick on the blue.
"I'll think about the title if I'm still here on Sunday night, and not before. I've just got to prepare for Saturday now. I've been practising hard this season and I'm getting results."
Semi-finals:
Friday: Robert Milkins v Ricky Walden
Saturday: Mark Williams v Mark Allen
