Ronnie O'Sullivan has apologised after walking out of his Maplin UK Championship quarter-final against Stephen Hendry on Thursday.
The former world champion was trailing 4-1 when he dramatically handed victory to his opponent after missing a red.
O'Sullivan said: "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."
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.
He said in a statement afterwards: "I had a bad day in the office.
"Anyone who knows me knows I'm a perfectionist when it comes to my game and 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.
"I wish I could have given Steven a better game and I'm sorry I didn't stick around to sharpen him up for his semi-final.
"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."
Hendry was visibly stunned as O'Sullivan shook his hand and walked out of the Barbican Centre arena.
"He just said he had had enough," said Hendry, who was awarded a 9-1 win. "I had no inkling anything was wrong."
Hendry's semi-final opponent in York will be either reigning world champion Graeme Dott or Steve Davis.
Hendry and O'Sullivan have clashed in the past, but the Scot said there were no longer any problems between the two.
"There has been no bad feeling between us over the last few days, there is no bad feeling between us anymore anyway," he said.
"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.
good feeling to be in the next round. 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."
World Snooker's Michael Ganley, the tournament director, confirmed O'Sullivan had conceded the match.
"I asked him if anything was wrong and he said 'no'. He's left the building and it has shocked us all," said Ganley.
"I've seen players concede at certain stages of the game before, usually through illness. It happens occasionally but it's very rare.
"Obviously he felt he couldn't continue, maybe for reasons we're not aware of, but he's forfeited the match."
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.
Maybe he should put his cue in the case and leave it there
John Parrott on O'Sullivan
John Parrott 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," said Parrott, a former world champion who is commentating on the event for BBC Sport.
"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."
A statement from World Snooker revealed spectators who had paid to watch the two sessions of the Hendry-O'Sullivan match would be offered tickets for Friday's semi-final between John Higgins and Peter Ebdon.
"It is made clear in literature relating to tickets that players appear circumstances permitting," read the statement.
"Spectators who bought tickets today will be able to watch the match between Graeme Dott and Steve Davis this afternoon and evening.
"They will also be offered tickets for tomorrow's snooker."
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What a t!t, is his life really that hard?