|Betfred World Championship|
|Venue: Crucible Theatre, Sheffield Dates: 31 July-16 August|
|Coverage: Watch live on BBC One, BBC Two, BBC Four and Red Button, with uninterrupted coverage on BBC iPlayer, BBC Sport website and BBC Sport app. Full details and times.|
Ronnie O'Sullivan and Judd Trump both face a battle to stay in the World Championship after falling behind in their quarter-final matches.
Five-time winner O'Sullivan trails Mark Williams 6-2, with defending champion Trump 10-6 down against Kyren Wilson.
Meanwhile, three-time winner Mark Selby leads Australia's 2010 champion Neil Robertson 11-5.
In the final best-of-25 tie, Anthony McGill has established a commanding 7-1 lead over Norway's Kurt Maflin.
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Trump went into Monday evening's session trailing 5-3 and searching for his best form.
And it proved to be a frustrating experience for the world number one, who twice reduced his deficit to a single frame but lost the chance to level the match by missing the final black in the 12th frame.
The trend continued as Wilson, who won his first ranking title at Trump's expense in the 2015 Shanghai Masters, grew in confidence.
A superb break of 94 - the highest in the contest so far - and another of 80, helped the eighth seed construct a notable lead as he reeled off the last three frames.
Trump now faces an uphill task when his encounter against Wilson concludes on Tuesday afternoon.
No first-time snooker world champion has retained the title since the tournament moved to its current venue in 1977.
O'Sullivan struggles against vintage Williams
While five-time winner O'Sullivan has already demolished Thailand's Thepchaiya Un-Nooh and beaten China's Ding Junhui, facing Wales' three-time champion Williams was a step up in class.
A high-scoring start saw the former champions, who both turned professional in 1992, trade frames with breaks of 70 and 101.
But after the Englishman briefly edged 2-1 ahead Williams assumed control, compiling breaks of 72, 56 and 130 as he won five frames in a row.
Selby and McGill dominate
Selby holds holds the upper hand in his quarter-final with Robertson, after dominating both of Monday's sessions.
The Englishman is now just two frames away from reaching the single-table format for the first time since winning his third world title in 2017.
Selby, who edged a marathon 58-minute opening frame, the longest in this year's tournament, established a 5-0 lead before the Australian hauled himself back into the match with breaks of 83, 66 and 65.
However, Selby maintained his fine form on Monday evening, winning the session 6-2.
In the battle of the qualifiers, Scotland's Anthony McGill already looks on course for a first Crucible semi-final after taking charge of his opening session with Norway's Kurt Maflin.
McGill had moved into the quarter-finals after staging a remarkable comeback to beat fellow qualifier Jamie Clarke 13-12 in a bad-tempered encounter.
Meanwhile the second and final round of coronavirus tests at tournament has returned no positive results.
There were 120 tests on players, officials and staff before play started on Monday.