Only John Higgins now stands between the Irishman and a second successive title. Higgins, cushioned by the 8-0 session whitewash which had given him a 12-4 overnight lead, completed a 17-9 victory.
If he picks up the pounds 220,000 winner's cheque, Higgins, who has won three titles from his seven finals this season, will take top place in the end- of-season rankings, thereby ending the eight-year reign of his fellow Scot, Stephen Hendry, as the world No 1.
Doherty won a fluctuating match of tight frames, many potting positional errors and few impressive breaks. His clinching 79 superseded Williams' 71 as the highest of four sessions of mediocre break-making, particularly in the context of the 52 centuries the championship has produced so far - four more than the previous record.
Doherty led 6-2 after one session but was 12-8 adrift midway through the third before breaking his seven-frame losing streak, three in black- ball finishes, with three consecutive successes of his own. At 41-0, it seemed as if he might finish Friday at 12-12, but Williams' winning 56 left him 13-11 adrift going into yesterday's concluding session.
Williams was attempting to become the first Welshman to reach the final since Terry Griffiths lost to Steve Davis 10 years ago. He was about to take a significant step in that direction when he overcut the green at close range in the day's opening frame with a three-frame lead there for the taking. He was given no second chance in that frame and did not score in the next after his initial 50, Doherty snatching it on the pink with a 49 clearance to level at 13-13.
Williams led 14-13 but when he was 25 behind in the 28th with one red remaining he was trapped in a snooker which led to his resignation after four failed escapes had seen his deficit grow to 41. He could summon no further resistance, totalling only 12 in the remaining three frames as Doherty's experience and match toughness told.
In his six events since February, Doherty has reached a quarter-final, four semis and a final. But at Sheffield, not quite in top gear, he has felt the title weigh heavily upon him. "It is a pressure. It's difficult to put into words. It's to do with not losing something you have which is so nice," he said.
It was consistency that made the difference in the other semi-final. The contrasting personalities of the 22-year-olds involved add piquancy to their developing rivalry. O'Sullivan has a colourful history and off- table life, a volatile temperament and the innate confidence of the instinctive genius. Higgins has nothing to declare but an excellence he applies himself unremittingly to maintain.
O'Sullivan's five finals this season have yielded four titles, but he is a virtuoso, not merely inconsistent but too impatient of his own imperfections. Even when - especially when - Higgins is struggling, he never loses his intensity of purpose as O'Sullivan tends to. "I didn't stick in there," the loser admitted.
Even so, he had a fleeting chance of getting back into contention when he took the opening frame of the morning on the black and went on to take four of the first five frames but lacked the steadiness or, it seemed, the intensity of purpose to maintain his revival.
After he added the second frame with a break of 69 there was an exchange of frames, both players making errors, which kept the gap at six before O'Sullivan, frustrated by misjudging a positional shot, launched into a blatantly inappropriate alternative. Despite more recklessness, he was somehow presented with a chance to clear with 37 for the black ball win that reined Higgins in to 13-8.
Going well with 55 in the 22nd, O'Sullivan looked like closing the gap to four until careless failure at a simple middle pocket red let Higgins in for a resolute 79.
Even more carelessly, O'Sullivan cast away the penultimate frame of the morning. With Higgins needing two snookers, O'Sullivan obliging fouled the black bridging over it and left the free ball from which Higgins cleared for 15-8.
Unpredictable as ever, O'Sullivan cleared with 75 from 0-46 to win the last frame before lunch, but the evening session consisted of a mere 20 minutes' action as Higgins passed the post with runs of 69 and 119. His nine centuries in this championship include three in four frames inbeating Anthony Hamilton in the last 16 and the first century hat-trick in the championship's 71-year history as he prevailed against John Parrott 13- 11 in a tough quarter-final match.Reuse content