Doherty's provisional number one status is now under threat

Doherty won only one of the last seven frames after resuming 5-3 up in the second round tie.

Ding Junhui remained on course to become only the third man to retain the title, beating Matthew Stevens 9-5.

Stephen Hendry held off Mark Williams to move into the quarter-finals, while Steve Davis beat Alan McManus.

Hendry failed to impress in his first-round win over Dave Harold, but recovered his form in the early session to move into a 6-2 lead and eventually won 9-6.

However, Welshman Williams won the next three frames before Hendry recovered his form to grind out an untidy win and set up a clash with Ronnie O'Sullivan.

I'm relieved to have won because I didn't play very well Ding Junhui

"Slowly but surely I think my form is coming back," said the 37-year-old. "I took advantage of Mark's bad play this afternoon but he was in great form tonight."

Ding, who will now face Peter Ebdon, admitted his busy schedule was beginning to take its toll as he overcame moments of poor concentration to beat Stevens.

"I'm relieved to have won because I didn't play very well," said Ding, who won three gold medals at the Asian Games in Doha before this event.

"I'm not feeling very stable just now. I didn't feel very confident before the match and I was a bit worried in the first session because Matthew played well."

And Doherty admitted he also failed to hit top form against Perry, who has reached the semi-finals for the last two years in this event and will now face John Higgins.

"Joe shut me out for three frames," said the 1997 world champion. "He played really well tonight, and I missed a few chances.

"I should have gone 6-3 up - and if I had it would have been different. But sometimes that's the way it goes, and it makes things pretty difficult."

Davis finally wrapped up his six-hour epic against McManus with a 46-minute final frame going his way.

"I feel as though I'm still pretty competitive and I'm enjoying the competition," said the 49-year-old Davis.

"I would have been disappointed to lose that match from the position I was in."