But that was as good as it got for the five-time world champion at Alexandra Palace as Allen responded to take six straight frames and advance to the semi-finals.

"I'm glad it is over. It was a bit of a tough match for me," O'Sullivan told the BBC.

"I don't make excuses. I had to show up. If I was a footballer I would probably have missed this game and they'd have put somebody else in for me.

"But in an individual sport you sometimes have to just get out there and do it.

"If I had pulled out there would have been murders, everyone would've been shouting and screaming.

"I just thought, Jjust turn up, give it a go'. I wasn't able to turn it around."

O'Sullivan insisted his illness was no worse than in his earlier win over Marco Fu.

And O'Sullivan was magnanimous in defeat.

"I'm struggling. I don't know what it is, whether it's a virus," O'Sullivan added.

"I've had it before and it's very difficult when you're getting dizzy spells.

"I gave it a go and wasn't good enough. You have to give people credit. It was a fantastic performance, fantastic game."

Allen took the second frame with a break of 65. That was followed by breaks of 115, 85 and 81, which gave Allen a 4-1 lead.

And breaks of 39 and 21 helped him to within one frame of victory.

Both players missed chances to take control of the seventh frame, before Allen grasped his opportunity to claim a notable scalp and his place in the semi-finals of the tournament which concludes on Sunday.

"I'm very, very happy, I scored really well," Allen told BBC 2. "I find against Ronnie I play more aggressive because you're going to get fewer chances."