Five-time champion Ronnie O'Sullivan faces a battle to stay in the World Championship after Mark Williams hit form to build a commanding lead in Sheffield.

Welshman Williams, seeking a fourth world title of his own, won five consecutive frames at the Crucible to establish a 6-2 overnight advantage in the best-of-25 quarter-final.

There was nothing to separate the veteran duo before the mid-session interval as they shared the opening four frames, which included an O'Sullivan century.

But O'Sullivan, who controversially questioned snooker's next generation after beating Ding Junhui on Sunday by saying he would need to "lose an arm and a leg" to fall out of the top 50, was second best on the resumption of play and left with plenty to ponder.

Opponent Williams edged a scrappy fifth frame before propelling himself into a healthy position with breaks of 72, 56 and a superb 130.

Earlier, Kyren Wilson moved into a 5-3 lead over defending champion Judd Trump.

Wilson drew first blood with a break of 74 in the opener but world number one Trump responded with scores of 85 and 76 in the next two frames to edge ahead.

A run of three successive frames then helped Wilson build a 4-2 lead before Trump reduced the deficit by taking the seventh.

However, Wilson was able to restore his two-frame lead with a break of 79 just before the interval to leave Trump trailing after the first session for the third successive match.

Mark Selby held the upper hand in his quarter-final with Neil Robertson after also establishing a 5-3 lead, although the advantage could have been even greater.

Selby edged a marathon 58-minute opening frame, the longest in this year's tournament, and then reeled off the next four frames to race 5-0 in front.

However, Robertson hauled himself back into the match by winning the final three frames of the session, recording breaks of 83, 66 and 65.

In the battle of the qualifiers, Anthony McGill already looks on course for a maiden Crucible semi-final.

The Scot dominated the opening session of his meeting with Norwegian Kurt Maflin and will resume on Tuesday holding a commanding 7-1 lead.