Mark Williams fell to a shock 9-7 defeat to Fergal O'Brien in the UK Championship in York, to bring his run of 48 consecutive wins in the opening round of ranking events to an end.
The opening session finished level at 4-4 with an out of sorts Williams snatching the final two frames on the colours after O'Brien had capitalised on his opponent's errors.
And the Welshman failed to find top gear in a drawn out encounter with O'Brien securing his place in the last 16 with the aid of a fluked blue in the 16th frame.
"I am very disappointed to go out playing like that," said Williams.
"It was 16 frames of total rubbish.
"In every tournament I play really badly at some stage and I'm getting sick of it.
"Other top players must watch me sometimes and laugh to themselves that I am world champion and world number one - I can play so badly at times that it is frightening."
O'Brien, who faces crowd favourite Jimmy White in the quarter-finals, admitted the win was a major boost after seeing his world ranking slip from ninth to 33rd.
"People thought I was dead they had not seen me on TV for so long," he joked.
"It is great to have beaten Mark but we were both struggling. If he had won, I would have let him off the hook but by the same token, he let me off the hook because I didn't play well."
Scotland's Stephen Hendry defeated John Parrott 9-5 to progress to the third round.
Hendry could not reproduce the vintage form that won him last week's British Open, failing to find his rhythm until the final two frames of the opening session where he constructed breaks of 73 and 128.
Parrott fought back to 7-5 with a break of 57 in the 12th frame, before Hendry won two scrappy frames to secure the win.
"It's a win and that's all you can say," said Hendry.
"I certainly didn't carry my form from the final in Brighton into this match.
"I knew I was in for a tough time, even at 6-2 up. We've had some great battles in the past and John has always been toughest when his back is against the wall.
"There's nothing wrong with my game - it was just one of those days where the object of the exercise was to win."
Ronnie O'Sullivan bounced back from the disappointment of losing to Hendry in the British Open final last week by building a commanding 7-1 lead over Ian McCulloch
O'Sullivan's 140 total clearance in the third frame puts him in the running for the highest break prize, and other breaks of 50, 55, 61, 52, 77 and 52 to move within sight of the third round.
Former champion John Higgins was struggling to remain in the hunt after losing the opening session of his second-round match against James Wattana 5-3.
Wattana stormed into a shock 4-0 lead with breaks of 60, 56 and 95 to race ahead.
But Higgins responded with runs of 55, 41, 42, 48 and 46 to reduce his arrears to 4-3, but missed out on the chance to level things up after missing the final red in the eighth frame.
Wattana squandered one chance to clear up, but took the frame at the second opportunity
to secure a two-frame lead.