Speed and ferocity are terms not normally used to describe exchanges in snooker. But Fergal O'Brien will testify to both last night as the tournament's third-seed Mark Williams riddled the Irishman in five straight frames to end his hopes of a place in the semi-finals of this year's World Championship.

Williams, leading 12-4, needs just one game from today's morning session to advance. On the adjoining table Belfast's Joe Swail continued his renaissance, playing his way into strong contention for a place in the semi-final. He currently stands with a comfortable four-frame advantage over England's Dominic Dale. Swail moved from a 4-4 first session score to win six out of the evening's eight frames for a comfortable 10-6 lead. O'Brien, however must hope for one of the most miraculous recoveries in snooker history to survive today's competition.

Williams led 5-3 going into the second session but soon posted his intentions, senior referee John Williams occasionally finding himself almost brushed aside as his compatriot hastily presented himself for each pot.

A break of 101 was followed by 112 for a 7-3 lead as Williams moved on at frightening speed. There was little O'Brien could do. Unusually, O'Brien had 53 per cent of the match-table time despite Williams leading 93. At 12-4 when the mid-session had ended, Williams, quite extraordinarily, had just 51 per cent of the time, ample illustration of his fasten-your-seatbelt style.

O'Brien stopped the charge after a 68 break to tie the frame, pocketing the re-spotted black for 10-4. But the respite was temporary, a break of 71 then taking Williams to the brink. The Dubliner now needs an almost impossible nine frames without reply to go through to his first World Championship semi-final. There were two or three occasions when the Irishman might have been able to employ damage limitation but jawing a few easy pots simply exacerbated the situation. Williams will now face either John Higgins or Andrew Hamilton.

Swail, more of a grip-and-rip type player than O'Brien, took to his task against Dale in much the same way as he had finished against 1991 champion John Parrott the previous day. In a much slower-paced match, the Belfast player took a three-frame lead at 7-4; allowed Dale come within two at 8-6 and then stamped his authority in a cautious but ultimately rewarding final frame for 10-6. Apart from Swail's century (119) in the opening morning exchange, he had little opportunity to really show his class as a break builder in the way he did in the 15th frame against Paul Hunter in the first round. His 141 still stands alongside John Higgin's score as the joint-highest break in the tournament so far, keeping him in line for a share of the £20,000 prize.

It was Parrott who put Swail's dangerous ability into perspective on Sunday. "There are players on the circuit who are an absolute nuisance to me. Every time I play him he plays his brains out. He played like a man possessed, his long game especially." The first player from Northern Ireland to figure in a World Championship quarter-final since Dennis Taylor in 1993, Swail has always been viewed as an authentic talent with his penchant for partying rather than practising making him occasionally brilliant but rarely consistent. His current advantage means that a semi-final place today remains a real possibility.

Needing only three frames from the nine on offer, Swail's chances of a first semi-final now is in his own hands.

In the early sessions of the other quarter-final matches, Jimmy White stands at 5-3 down to ninth-seed Matthew Stevens. White matched Stevens frame for frame to go 3-3, Stevens posting a 109 in the fifth, before compiling breaks of 60 and 63 to leaving the 37-year-old trailing by two frames.

The 1998 winner John Higgins stood less on ceremony. His early efforts against Andrew Hamilton left him 5-3 in front, proving the accuracy of Ken Doherty's opinion that Hamilton would have to raise his level of play if he wishes to advance. Hamilton, however, impressively stayed in contact soberly nicking the final frame 62-61 after Higgins had shot to a 61-0 lead.