'Struggling' Ronnie O'Sullivan dumped out of Masters by Mark Allen as Wilson thumps Williams

The defending champion won the opening frame with a break of 75, but his opponent won the next six.

It was a win that was highly unexpected especially following O'Sullivan's 6-0 demolition of Marco Fu in the first round at Alexandra Palace on Monday.

The second quarter-final of the day was also a one-sided contest as Kyren Wilson completed a 6-1 win over Mark Williams.

Williams - the 1998 and 2003 Masters champion - could not replicate the level of form that saw him overcome world champion and world number one Mark Selby 6-5 in the first round on Sunday.

Wilson won the first four frames before a knock of 63 carried him 5-0 clear.

Williams prevented the whitewash by winning the sixth frame before a closing 76 from Wilson saw him progress to the last four. He will face Judd Trump or Shaun Murphy for a place in the final.

Northern Irishman Allen - a former UK Championship finalist in 2011 - enjoyed breaks of 65, 115, 85, and 81 against O'Sullivan to secure his place in the last four against John Higgins or Ryan Day on Saturday.

"I didn't give him much to really challenge him," said O'Sullivan afterwards. "The crowd always want to see a close game, but sometimes it's just not there."

"I'm struggling, I don't know what it is, a virus or something and I was having dizzy spells," he said.

"It was a tough match for me. I can just rest up and get ready for my next event. Mark deserved his win, I didn't give him much to make him struggle. You have to give people credit and he had a fantastic performance.

Although the world number two had revealed he had been feeling ill prior to the previous round, he refused to attribute his loss to that, saying, "This isn't a physical game, it's a mental game...no excuses, you need to tick all the boxes."

It was O'Sullivan's first defeat at the sport's biggest invitational event since he lost 6-1 to Neil Robertson in the 2015 semi-finals.