Amateur James Cahill is five frames away from producing one of the all-time sporting shocks at the Crucible.
Cahill, the first amateur to play at the World Championship, leads Ronnie O'Sullivan 5-4 at the halfway stage of their first-round match in Sheffield.
World number one O'Sullivan looked out of sorts throughout and must pick his game up when play resumes on Tuesday morning.
Cahill rarely looked in awe of his fancied opponent, who is chasing a sixth world crown.
John Higgins, hunting a fifth title, is safely through to round two after beating Mark Davis 10-7, rattling in breaks of 100, 135 and 132 along the way.
In the morning session, three-time champion Mark Selby had to fight back in the opening instalment of his first-round clash, recovering from 5-1 down against Zhao Xintong to close out at 5-3.
The Leicester man is desperate to find some form in Sheffield, having made first-round exits in the same tournament last year, this season's UK Championship and at other notable events during the campaign, including last month's Players Championship.
While Selby was toiling, Shaun Murphy was condemning Luo Honghao to the heaviest defeat in tournament history.
In winning 10-0, Murphy became just the second man to record a Crucible whitewash, while Luo's 89 points were a record low.
Murphy now faces Neil Robertson, who won his first-round clash 10-1, and hopes he can make the most of his late-season form.
"After the season I have had, never in my wildest dreams did I think I could come here and win 10-0," Murphy is quoted as saying by World Snooker's official website.
"Going out this morning, it was tough to try to put that potential 10-0 to the back of my mind and try to play the right shots.
"This has been the worst season of my life, I have been in a daze. Yesterday I felt like Shaun Murphy again. In a way I'm looking forward to this season being over, but while I'm here I'll give it my best."