RONNIE O'Sullivan was warned for swearing and title-holder Shaun Murphy suffered a nightmare in the two showpiece quarter-finals of the 888.com World Championship at the Crucible Theatre yesterday.

The afternoon matches involved a quartet who have won the six renewals of the tournament since 2000 and O'Sullivan ended the first session level at 4-4 with Cwm's Mark Williams.

After the opening two frames had been shared, each player making a single visit to the table count, 'The Rocket' vented his frustration in the next.

A poor positional shot prompted an audible obscenity from the world number one, earning him snooker's equivalent of the yellow card from referee Jan Verhaas. Any further outburst during the rest of the match would result in Williams being awarded a frame.

O'Sullivan missed an easy brown and went 2-1 down before the Welshman extended his lead with a break of 67, but after the interval the 2001 and 2004 champion showed more of his quality.

He trailed 4-2 when Williams, after potting the opening red in frame seven, knocked his head on the table in despair as he ran out of position and O'Sullivan took full advantage.

Breaks of 45 and 79 ensured the 30-year-old from Essex was all square going into this morning's second stint.

On the other table, Murphy, aiming to become the only first-time champion to have successfully defended the crown, found himself 7-1 down to Peter Ebdon.

The South Yorkshire crowd, firmly behind Rotherham-based Murphy, was left stunned as he simply never got going until the last frame of the session when he made a decisive 64.

But despite his huge advantage, there was still disappointment for 2002 champion Ebdon as he missed the chance of a maximum 147 break in frame seven.

After potting 12 reds and 12 blacks, the Dubai-based 35-year-old left himself a tricky pot into the middle, bridging over another ball, and agonisingly saw it hit the jaw of the pocket.

Earlier, Marco Fu, the surprise package of the tournament, continued his fine form by taking a 5-3 lead against Ken Doherty.

The Hong Kong player, who had to qualify for the Crucible, ousted Scottish duo Alan McManus and Stephen Maguire to reach the last eight and his fine cue-ball control was on show again.

Doherty, the 1997 champion, went into the match as hot favourite after his impressive 13-8 victory over Matthew Stevens and made the ideal start by winning the first two frames.

But then Fu found his rhythm and reeled off five in succession, completing the spree with a break of 121 that was his third century of the competition.

It meant the last frame of the session was vital for Doherty to avoid falling too far behind, and the Irishman responded by clinching it after Fu had missed the pink into a middle pocket.

Graeme Dott, the 2004 runner-up, opened a 5-3 advantage over Australia's Neil Robertson in a contest that featured plenty of mistakes from both men.

The Scot went 4-1 up thanks to a break of 73, but a 109 by the qualifier from Melbourne put him back within one frame before Dott secured the last.