By Phil Harlow
BBC Sport at The Crucible
Joe Perry battled through a bout of shocking form to send defending champion Mark Williams out of the tournament after a 13-11 win.
Perry celebrates his shock victory
Williams had launched an astonishing comeback from 10-6 down overnight, reeling off five consecutive frames to lead 11-10 as Perry's form vanished.
But Perry composed himself, stopping the rot with a 55 clearance to level.
With the pressure on, a 53 put Perry within one frame of victory and he held his nerve with a 82 in the last to win.
Williams made no excuses after his defeat.
"I came back into it well and really put the pressure on him but he responded well," said the Welshman, who became a father last week.
"I thought I had a chance of winning but I didn't play well all through the match if I'm honest.
"I can't put it down to anything other than Joe playing better and holding his nerve when under pressure."
Perry admitted he had struggled to handle the pressure of a big lead.
"When Mark took the lead, I was playing for pride rather than having expectations on me," said Perry, who had fallen 11-10 behind before rallying.
"It's a massive win for me even if Mark didn't play to his full potential.
"It has to be the biggest win of my career, beating the world number one and reigning world champion at The Crucible under extreme pressure."
M Williams (Wal) 11-13 J Perry (Eng) 21-59, 29-70, 66-6, 65-22, 0-145 (145), 62-21, 1-77 (76), 73-76 (70 Williams), 61-62 (60 Williams), 0-72, 0-90, 38-62 (60), 60-0, 83-0 (83), 24-64, 58-21, 86-8 (52), 64-52, 69-56, 77-14 (52), 72-12 (67), 49-87 (55), 5-76 (53), 1-82 (82).