Working on the assumption that lightning doesn't strike twice, Joe Swail's interest in the World Championships at the Crucible looks perilously close to an end.

The hole in which he finds himself against 10 to 11 bookies favourite Ronnie O'Sullivan is now even deeper than the one Mark Williams put him in during the second round.

Swail now trails 15-9 with O'Sullivan needing two frames today for a place in the final. Swail's path to his second consecutive semi-final has been, if anything, arduous and fretful.

Against Sean Storey in the first round the 31-year-old was two frames down with three to play before he overcame the 99th-ranked English player 109.

He then trailed world number one Williams 11-7 before a remarkable 13-12 comeback. Swail was subsequently extended to 13-11 by his practice partner Patrick Wallace.

Looking to quickly get a grip on the match, Swail, still the front runner for the £20,000 highest break prize with 140 and already guaranteed £73,000 for reaching the semi-final, began yesterday's play watching O'Sullivan relentlessly strengthen his lead position.

The 16th seed was forced to sit out the first 20 minutes as O'Sullivan rattled off two peerless century breaks, of 108 and 119, for a threatening 8-2 advantage.

They were O'Sullivan's fifth and sixth centuries of the tournament. It could not have been more difficult for the Belfast player, who refused, despite missing several relatively simple pots, to let the match unravel before him.

Twice coming back from six-frame deficits, Swail probably missed too many routine opportunities to pressurise O'Sullivan but did show a glimpse of his `A' game with a 92 clearance before moving to 10-6 down.

That left O'Sullivan with the possibility of winning a place in the final last night. Needing seven frames from the eight available, he opened by taking four of the first five.

O'Sullivan missed a difficult black lined up for a 140 break in the 18th frame which would have equalled Swail's and the tournament's highest so far.

From 13-7, O'Sullivan hit a 79 break for 14-7 before Swail pulled two back - including a 138 break - in frame 23. But O'Sullivan responded sharply, ending with an 84 to go 15-9 ahead overnight.

John Higgins ended his second session level against Wales' Matthew Stevens in what is becoming a gruelling duel between two players in championship form.

From the beginning both produced their sixth century breaks of the competition, Stevens with a 133 in the 10th frame and Higgins a 126 in the 13th.

"I'm definitely in it to win it," said Higgins. "I think that if you do win something like the world championship two or three times you can be classed as a true great in the game.

"But to get my name on it a second time would mean the world to me - probably more than the first one."

Higgins, the 1998 champion, went into the session 5-3 ahead but Stevens won four of the eight frames to level 8-8. The match resumes this morning.