Ronnie O'Sullivan has apologised to his opponent Stephen Hendry and fans after walking out of his Maplin UK Championship quarter-final on Thursday.
O'Sullivan leaves the arena after handing victory to Hendry
The former world champion was trailing 4-1 when he dramatically conceded the match after missing a red.
O'Sullivan said: "Anyone who knows me knows I'm a perfectionist.
"Today I got so annoyed with myself that I lost my patience and walked away from a game that, with hindsight, I should have continued."
O'Sullivan was 24 points ahead in the sixth frame when he missed an attempted pot and immediately declared the match over, shaking the hand of Hendry and referee Jan Verhaas before walking out of the Barbican Centre arena.
At this moment I am feeling disappointed with myself and I am hurt and numb
"He just said he had had enough," said Hendry, who was awarded a 9-1 win. "I had no inkling anything was wrong."
O'Sullivan blamed his decision on a "bad day in the office".
"I wish I could have given Stephen a better game and I'm sorry I didn't stick around to sharpen him up for his semi-final," he added.
"I'm also really sorry to let down the fans who came to see me play - it wasn't my intention to disappoint them and for that I am truly apologetic.
"At this moment I am feeling disappointed with myself and I am hurt and numb.
"But I am a fighter and I will be back on my feet fighting stronger and harder than ever very soon."
He just said he had had enough and wished me good luck
Hendry and O'Sullivan have clashed in the past, but the Scot said there were no longer any problems between the two.
"He seemed in good form beforehand and we were chatting back stage," Hendry said. "There is no bad feeling between us anymore anyway.
"Ronnie's obviously got his reasons and I'm not going to criticise him.
"He just said he had had enough and wished me good luck for the rest of the tournament. Only he knows what he feels inside.
"I can't criticise someone else for that, but I've never seen anything like it. It's just bizarre."
It is very rough on the people who have paid good money to watch him
Hendry will play Graeme Dott in the semi-final, his fellow Scot a 9-6 winner over Steve Davis.
"I'm not going to lie, it's a good feeling to be in the next round," Hendry added. "But I was enjoying the match, playing well and obviously I would much rather have won properly.
"If he's got problems I feel for him, but I've never seen anything like it in my life."
Dott, hearing the commotion on the next table, was similarly puzzled.
He said: "I actually thought there was a fight - I heard someone in the crowd shout out quite aggressively, something like 'get a grip, Ronnie'.
"If Ronnie is medically depressed or has problems I would genuinely feel sorry for him and hope he gets better.
"But if he just can't be bothered and there's nothing wrong with him, I think that is obviously wrong.
"It is very rough on the people who have paid good money to watch him and people who have bet on him to win the tournament. It is them I feel sory for."
World Snooker's Michael Ganley, the tournament director, confirmed O'Sullivan had conceded the match.
Spectators who had paid to watch the two sessions of their clash were allowed to watch the Dott-Davis match instead, which caused disruption as spectators moved around to see the action on the other table.
They were also offered tickets for Friday's semi-final between John Higgins and Peter Ebdon.
Maybe he should put his cue in the case and leave it there
John Parrott on O'Sullivan
O'Sullivan, a former world champion and world number one, is one of the game's most charismatic figures.
But the 31-year-old from Essex is also a troubled soul, battling depression and threatening to quit snooker on numerous occasions.
Former world champion John Parrott, an analyst for BBC Sport, thinks O'Sullivan should quit the game for good if he is unhappy.
"Ronnie has got a bit of time now over Christmas to go away and reflect on what he has done here, then maybe he can come back with a fresh approach in the new year," Parrott said.
"But if the game is making him miserable and he really doesn't want to be there, then maybe he should put his cue in the case and leave it there."