It might be a vintage year for breaks of over 140 at the Crucible in Sheffield - four so far from a total of 21 since 1977 - not so for aspirant and current world champions. Yesterday was not a good day for title holder Ken Doherty or Ronnie O'Sullivan.

O'Sullivan is in deep trouble. Doherty's nerves are sinply frayed.

No lasting damage to the Irishman but a recovery was needed and Doherty possessed the character to provide. He still chases his Welsh opponent Mark Williams 11-13.

Overnight, however, Doherty will wonder how he allowed seven frames in succession slip by him, how a 6-2 advantage yesterday morning was horribly transformed into a 8-12 disadvantage before a quite spectacular turnaround brought the Irishman back into the match.

The Dubliner will be trying to fathom how he allowed Mark Williams gather a rythmn and take hold of this match in a most impressive way, how his opponent won the middle section of the 16 frames and how the champion was forced, at one stage, to watch Williams pot balls for over two hours without winning a game.

Doherty's rich talent occasionally escaped him but in a match dominated by vicious swings of fortune, something was lost and regained by the Irishman as he searched for and found the form that toppled Stephen Hendry last year. Doherty's fortitude and mental toughness, last night, kept him in the World Championship.

In today's nine-frame session Doherty needs six to progress with Williams needing just four to meet either O'Sullivan or Scotland's John Higgins.

Doherty's play in the morning session was littered with errors which frequently allowed the young Welshman back to the table. He began the day well, initially building from his 6-2 overnight lead to take the match to 85. But even then there were signs that the session was drifting.

In frame 12, Doherty comfortably led until he potted the white letting Williams in at 49-12 to take the frame. In the 14th frame Doherty was rattling away strongly before he again miscued an easy black on a break of 40. Williams was again able to muscle his way onto the table and claim the game 79-40 to trail 8-6.

That was to be the beginning of the seven-frame run for the lefthander, who again picked up when Doherty missed a green into the yellow pocket in what looked like a clearance run. Williams greatfully lapped up the opportunity to snatch the remaining 25 points and reduce the lead by one game at 8-7.

The last morning frame also brought difficulties for Doherty as again he potted the white ball off a red at 33 when he was running up a frame winning break. Williams then created the opening to take the match to eight games each.

The losing streak lasted until frame 21 at which stage Doherty trailed 8-12 . While some may have questioned his confidence, the Dubliner courageously hit back three times for 11-12 before Williams got in with a long red to the green pocket to end the session 11-13.

In the other semi-final, crowd favourite O'Sullivan was forced to watch Higgins dominate their second session to a almost absurd degree. The match continued yesterday with the players tied at 4-4 before the young Scot effectively blew `The Rocket' off the table with a remarkable eight-frame run to take his advantage to 12-4.

Number three seed Higgins, who has been the most consistent player of the season so far by reaching six finals and winning two - the German Open an the British Open - compiled a run of breaks of 50, 70, 79, 84, 79 and 97.

Early lapses from O'Sullivan allowed Higgins gather momentum before he constructd his own confident finish to the session.

During the four final frames O'Sullivan, quite astonishingly, was able to generate only two points in total. In the 13th and 14th frames he did not pot one ball.

The mercurial number five seed now has a mountain to climb if he is to even take this match to the final session this evening. Another five frames in this morning's sesion could launch Higgins into his first world championship final.