Ronnie O'Sullivan branded himself "awful" despite keeping alive his hopes of a record-extending eighth UK Championship crown.
The world number one beat China's Zhou Yuelong 6-5 in a gripping quarter-final in York.
O'Sullivan led 4-1 but slipped 5-4 down, before winning the final two frames for a fine comeback win.
"I felt a bit embarrassed at times - it was just desperation I started to play all right," O'Sullivan told the BBC.
"I was just going through the motions and plodding away. I was just so frustrated with how poorly I played, there really was no pressure on me.
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O'Sullivan, 47, started with a superb break of 125 - the 1,267th century of his career - as he laid an early marker of intent to 25-year-old Zhou, an opponent he beat 6-0 in their previous two meetings.
A break of 68 followed as Zhou failed to pot a ball in the opening two frames, before a run of 47 saw the 39-time ranking event winner extend his lead.
O'Sullivan, appearing in his 20th UK quarter-final in 30 appearances at the tournament, used sandpaper to rectify a problem with his cue tip in the fourth frame.
His brief loss of concentration allowed Zhou to pull a frame back, but a composed 60 break looked to have put the seven-time world champion back on course for victory.
Zhou bravely rallied to win four frames in a row, but O'Sullivan made his vast experience count and held his nerve to close the match out in style with breaks of 77 and 122 to reach a 12th UK semi-final.
"Ronnie was fantastic in that last frame - that's why he's world number one," said Zhou.
"He just played perfect in the last two frames, but I'm proud of the way I played."
O'Sullivan will play Hossein Vafaei on Saturday in the first of this year's semi-finals.
Iran's Vafaei battled back from 4-3 down to beat China's Zhang Anda courtesy of three century breaks.
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