The world champion and world number one Mark Williams is out of the Benson and Hedges Masters, beaten six frames to five by Dubliner Fergal O'Brien.

O'Brien's debut success at the Wembley Conference Centre puts him through to the quarter-finals against fellow Irishman Ken Doherty or Anthony Hamilton.

But it was touch and go for the 28-year-old - he had to fight back from a 5-4 deficit to claim the shock victory.

The demise of Williams means four former Masters winners have failed to qualify for the last eight: Williams, Ronnie O'Sullivan, John Higgins and Alan McManus.

In addition, defending champion Matthew Stevens also lost in the second round of the �650,000 event.

A break of 53 finally helped clinch O'Brien's ticket to the next stage as he fought back from 5-4 down.

But it would have been unjust if O'Brien had not prevailed. He repeatedly outscored Williams including a century in frame three and 99 in the penultimate frame.

"It was important to get a good start because if you let Mark get away then he just goes from strength to strength," said the former British Open champion.

"I lost a couple of silly games but I kept going and kept playing the same way," added 28-year-old O'Brien.

Williams had few complaints about his defeat, even though he dragged himself back from 3-1 and 4-3 down, before unluckily snookered himself in the decider.

"I had a bit of bad luck at the end," agreed Williams. "But I didn't really deserve to win.

"I wasn't doing anything right from the start. Perhaps, I've been over-practising. I'm certainly not playing like a world champion or world number one.

"The only reason I'm still at the top of the rankings is because everyone else is losing."