The two favourites, Judd Trump and Ronnie O’Sullivan, are fighting to stay in the World Snooker Championship as they trail Kyren Wilson and Mark Williams respectively.
Trump, the defending champion and the bookies’ favourite going into the quarter-finals, is 10-6 down after the first two sessions, as Wilson, who has reached the last eight at the Crucible five times, looks to reach his first final.
O’Sullivan, the second favourite, was outplayed by the three-times champion Mark Williams, who leads 6-2 with two more sessions to come on Tuesday.
The third favourite, Neil Robertson, is also in trouble, trailing Mark Selby 11-5 after Selby had won the first five frames.
Trump must come from behind to keep alive hopes of retaining his crown. The world No 1 is seeking a remarkable seventh ranking event success of the season but, trailing 5-3 from the opening session, was unable to level as the pair initially traded frames.
Wilson responded to Trump’s knock of 73 with a break of 65 to maintain his two-frame lead. Trump, who beat John Higgins in last year’s final, again reduced the deficit to a single frame but then blew the chance to level at 6-6 by missing a costly final black.
The pattern continued as Wilson ensured he could not trail overnight with a superb 94, the highest break so far of a century-free contest, to go 8-6 ahead. He then pulled away, taking the final two frames to leave Trump facing an uphill battle on Tuesday.
Williams, seeking a fourth world title, won five consecutive frames 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.
Williams edged a scrappy fifth frame before propelling himself into a healthy position with breaks of 72, 56 and a superb 130.
Selby held the upper hand in his quarter-final with Robertson after 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.
But Selby resumed his dominance in the evening session, which he took 6-2 to extend his lead to 11-5 as he, like Williams, chases a fourth world crown, three years after his last.
In the battle of the qualifiers, Anthony McGill put himself in a strong position to reach a maiden Crucible semi-final. The Scot, who had come through final-frame deciders against Jack Lisowski and Jamie Clarke, dominated the opening session of his meeting with Norwegian Kurt Maflin and will resume on Tuesday holding a commanding 7-1 lead.
Maflin could not reproduce the excellent snooker that had seen him defeat David Gilbert and Higgins in the first two rounds.