World champion Graeme Dott bowed out and missed the chance to face O'Sullivan in the semi-finals of the Northern Ireland Trophy tomorrow.
Last season's Crucible winner was beaten 5-2 by on-song Welshman Dominic Dale.
The world number four admitted: 'Dominic played well. I had a nightmare start to the match.
'He should have been 4-0 up. It's not that I played bad, but I didn't do too much wrong.'
Victory would have seen Dott set up a repeat of last season's World Championship semi-final with O'Sullivan.
But with Dale continuing his fine form in Belfast this week, Dott took defeat in his stride.
'I still thought I could come back, but Dominic had a good start and it was always going to be hard to come back,' added Dott.
Dale won the opening frame with a 61 break and then went 2-0 ahead by winning a scrappy second frame.
It was soon 3-0, largely thanks to a run of 40, before Dott pinched the fourth frame to trail 3-1 at the interval.
Dott reduced his arrears further by winning the first frame after the resumption thanks to a break of 43, but Dale was not to be denied.
The 1997 Grand Prix champion produced a tournament high break of 137 to go 4-2 ahead, before winning the mach with a wonderful pressure clearance.
Dott got in first and made a 62 break, but Dale kept his cool to sink a 65 run and progressed to the last 4.
'At 3-1 and 3-2 up I started to lose my focus and my edge was going a little bit,' said Dale.
'But my form is superb and making a break of 137 was a great way to go 4-2 up.
'He (Graeme) left me a horrible red in the seventh frame, but I struck it beautifully and made a good break to win it.
'This is the best I've played in a tournament - and it's only because I've started to believe in myself.'
