Ronnie O'Sullivan suffered his first defeat since last April, leaving the defence of his Champions Cup title in the balance.
O'Sullivan went down 5-4 to Dubliner Ken Doherty at the Brighton Centre, giving Doherty outright lead of Group A.
It was O'Sullivan's first defeat in seven Champions Cup appearances stretching back to last year's tournament and he must now beat Peter Ebdon on Thursday to make sure of his place in the last four.
Barring a disaster against Jimmy White, former world champion Doherty is almost assured of a trip to the penultimate round.
He achieved victory the hard way though - coming back from 3-1 down after being blitzed by the "Rocket".
O'Sullivan, who finished last season by winning the World Championship and Premier League, made breaks of 87 and 80 as he took control.
But Doherty, a 5-2 winner in his opening game against Ebdon, recovered to level at 3-3.
Even when O'Sullivan regained the initiative, the Dublin cue man fought back with a clearance of 45 to level at 4-4 before taking the decider 101-4 with a 59 break.
Doherty said afterwards: "I have never been one to throw in the towel, but there is a huge difference between how I feel nowand how I was 18 months ago."
O'Sullivan, who opened his Champions Cup campaign with a win over White, said: "If you keep digging in like Ken did and never give up, then things will go in your favour.
"He made a couple of brilliant clearance and also played some good safety as well."