Peter Ebdon beat Ronnie O'Sullivan 6-2 to reach the semi-finals of the Citywest Irish Masters in Dublin.
Ebdon frustrated O'Sullivan into conceding with just 30 points on the board
O'Sullivan won the first frame but promptly lost the next five as Ebdon took control of the match.
O'Sullivan did not help his cause by walking out of the arena midway through frame four when Ebdon led 30-0.
The defending champion's actions lost him the frame - and Ebdon soon extended his lead to set up a clash with Quinten Hann, who beat John Higgins 6-4.
O'Sullivan defended his decision to walk out midway through frame four, insisting: "I expected Peter to clear up.
"I would have been surprised had he missed the reds."
Asked whether he felt his actions had let down his supporters, the former world champion answered: "I'm sorry if I disappointed the crowds.
"But sometimes you have got to do what you have to do. Opinions are like backsides - everyone has one.
"I didn't play very well, Peter played more solid and deserved to win.
"But I was disappointed I lost the game because I was enjoying myself out there. Some days you are the bug and some days you are the windscreen.
"Today I was the bug and I got smashed. But it's good to be able to sit here and still be upbeat after losing 6-2."
Ebdon said: "Players are entitled to concede whenever they want. It doesn't bother me if they do that. It just means you get to your interval cup of tea sooner.
"Ronnie missed a couple of balls and perhaps he was disgusted with himself - I don't know."
Hann began with a break of 111 against Higgins before quickly opening up a 3-0 lead.
The Scot hit back to reduce the deficit to just one frame but could never level the match as Hann sealed a match against Ebdon.
Hann, who becomes the first Australian to reach a ranking semi-finals since Warren King in 1990, admitted his nerves had been getting the better of him.
"If it had gone 5-5, I don't think I'd have won," said the world number 15.
"And I would have been sick."
Higgins was left to rue a missed pink in the eighth frame, which allowed Hann to take the frame and go two up with three to play.
"My son would have potted it," said Higgins. "I just lost concentration because I thought it was going to 4-4.
"I've just got to keep going and hopefully my next title is just round the corner," added the Scot.