Northern Ireland's Joe Swail showed nerves of steel to edge out former champion Mark Williams 10-9 in their first-round match at The Crucible.
Swail reached the semi-finals in Sheffield in 2000 and 2001
Swail, who had trailed 4-0 early on, took the first frame of the evening session to level at 5-5 before Williams went ahead with an 85 break.
But Swail refused to crumble as he hit back with a superb 114.
Williams led 8-6 and 9-8, but crucial misses allowed Swail back to make an outstanding break of 64 in the decider.
The defeat was the latest in a hugely disappointing season for Williams, who could now be under pressure for his top 16 place.
The Welshman showed his frustration on several occasions, slashing his cue through the air and leaning on the table with his head in his hands.
"I just didn't play at all - he was the better player all day and deserved to win," said Williams.
"I just wasn't there, I didn't perform. I played like a fish!
That's probably one of the best wins I've ever had
"I'm disappointed with the way I've performed over the whole season more than anything. It's probably the worst season I've ever had."
Swail, who will face Matthew Stevens in the second round, had to do things the hard way in the final frame as he ran out of position on a break of 64 with just one more pot needed.
A tense safety battle ensued as Williams battled to stay in the tournament, but a long red rattled in the jaws and a relieved Swail sealed the win.
"That's probably one of the best wins I've ever had," said Swail.
"Everyone was saying he's had a bad season and I should beat him, but that's rubbish.
"He's one of the best players in the world and won the world title twice, so it was never going to be easy. I'm so delighted with the result."
Mark Williams (Wal) 9-10 Joe Swail (NI)
64-39 68-23 80-36 72-14 29-66 (59) 34-78 (53) 26-69 (62) 22-71 (71) 75-58 (Williams 52) 38-74 90-39 (85) 0-114 (114) 78-1 55-42 (71) 52-63 0-69 87-34 1-71 (69) 0-121 (65, 56)