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YORK - Former snooker world champion Ronnie O'Sullivan dramatically walked out of his UK Championship quarterfinal against Stephen Hendry after just five frames today.

O'Sullivan, 31, was trailing 4-1 to Hendry when he decided to concede the match after missing a pot.

The two-time world champion shook hands with Hendry and referee Jan Verhaas before immediately leaving the Barbican Centre arena.

Scottish star Hendry advanced to the semifinals as a default 9-1 winner and will play either Steve Davis or Graeme Dott on Saturday.

According to World Snooker officials, O'Sullivan, who won the UK Championship in 1993, 1997 and 2001, did not want to hold a press conference to discuss his bizarre disappearing act.

Hendry admitted he had never experienced anything like it in his long and illustrious career and told BBC television: "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.

"He went for a silly red and missed it and I was thinking 'keep going' and suddenly he shook my hand.

"I had no inkling there was anything wrong, he knows what he's doing.

"I've been on the end of a few thumpings from him recently and so I was enjoying the game.

"I'm delighted to be in the semifinals but it's just bizarre.

"If he's got problems I feel for him but I've never seen anything like it in my life."

Tournament director Michael Ganley confirmed O'Sullivan had forfeited the tie and added: "Basically, Ronnie has 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.

"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."