Mark Allen will face Mark Selby over the best of 35 frames for a place in the final of the Cazoo World Championship after a hard-fought win over qualifier Jak Jones.
Allen was far from his fluent best and made just one break over 70, but took advantage of a nervy display in the final session from debutant Jones to secure a 13-10 victory.
Selby, meanwhile, had it much easier as he beat John Higgins 13-7 in his quarter-final
"It was never in doubt was it?" a relieved Allen joked after reaching the last four at The Crucible for only the second time and the first since 2009.
"It was a complete slog to be honest. The first session was really good but I was awful after that and it was just a matter of digging deep, trying to stay as positive as possible and trying to get 13 frames. That's what I did."
Asked what had been the deciding factor, Allen told the BBC: "The safety (department) in the end. We both missed balls.
"I felt like as the match went on he was getting more edgy but I wasn't playing well enough to capitalise, so it was just a matter of staying patient and taking my chances when they came.
"It wasn't pretty, I'm sure it wasn't pretty to watch but I couldn't care less. I'm in the semis.
"I don't mean this to sound bad to Jak, but that was like a three or four out of 10 performance. That really wasn't good. But mentally I was like a 12 out of 10 and ready for anything.
"Sometimes that's more important and that's a match I would have lost a few years ago."
With the contest finely poised at 8-8, Jones won the opening frame on Wednesday with a break of 77 before contributions of 66 and 64 from Allen edged him back in front as Jones missed a number of simple pots.
To his credit, the Welshman composed himself quickly to draw level with a break of 124 which was a total clearance in every sense as he deliberately potted the white along with the final black.
However, Allen took the next frame after twice trapping his opponent in costly snookers and a break of 64 then moved the left-hander to within one of victory at 12-10.
Jones was left needing two snookers to keep the match alive and got them both before playing a poor safety on the green, but was handed a lifeline when Allen failed to pot it into the yellow pocket.
That proved to be a short-lived reprieve, though, Jones leaving Allen a thin pot on the brown which he duly pocketed before adding the blue to seal victory.
Jones, who had beaten Ali Carter and former champion Neil Robertson to reach the quarter-finals, said: "I suppose it's a decent tournament but so disappointed with how I played in this match. It was just a terrible match.
"(I was) missing easy balls, which I haven't really done for the whole tournament, and then I suppose the occasion got on top of me a little bit."
On the other table, Anthony McGill and Si Jiahui resumed at 4-4 and shared the eight frames of their second session to remain locked together at 8-8.
