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Matthew Stevens completed the remarkable turnaround in his quarter-final against Ryan Day as he clinched a 13-5 victory.
The two-time former beaten finalist at the World Championship trailed 5-2 at one point on Tuesday but won 11 frames in a row to advance - nine on Tuesday night and two on Wednesday morning - as Day's game deserted him.
Stevens will face Neil Robertson or Ronnie O'Sullivan in the semi-finals at the Crucible, beginning on Thursday.
Stevens did not need to find anything approaching his best form to fend off his good friend, who began the match brightly but then saw his performance deteriorate dreadfully.
The flow of the match changed completely in last night's middle session, making a Stevens victory an inevitability.
Stevens said: "Last night was very strange.
"Both of us struggled, and it was very tough for Ryan. I don't know what was happening to be honest.
"It was tough to play against. Sometimes you've just got to dig in there and I managed to do that a bit better.
"It was very disappointing because it was such a good start. I made an 80, then he made two centuries, and I thought it was going to be a fantastic match.
"But snooker's not all about potting and making breaks, and sometimes you do struggle and you've got to stick in there."
Seeing Day suffer was awkward on Stevens, who said: "It was tough. But we're here to try to do a job.
"You've got to be fearless and get in there and try to win, and I managed to do that, but it was a disappointing match."
Now a sixth World Championship semi-final and a first since 2005 awaits the 34-year-old, who has revived his career after a slump which saw him fall outside the top 16.
"I'm so excited," Stevens said. "It's been a long time."
Beaten Day, who beat Ding Junhui and Cao Yupeng to reach the last eight, was at a loss to explain his slump in the match.
He said: "I played well in the first session but last night was the undoing. It was just a shambles.
"I just wasn't there. I don't really know why. Towards the end it was just embarrassment. I just wanted to get out of there.
"It was a good opportunity to push on and make a big impact these two weeks, but all the good work I'd done was undone in one session last night."