Ken Doherty believes Ronnie O'Sullivan can still challenge for more World Championship titles, despite his shock first-round exit meaning that time is running out for the five-time champion.

O'Sullivan was the favourite to land a sixth world crown in Sheffield after winning five titles this season to reclaim the world number one ranking for the first time since 2010, but looked out of sorts throughout a 10-8 defeat to 750-1 outsider James Cahill.

The 43-year-old admitted a combination of illness and ongoing insomnia had left him feeling "absolutely exhausted" as he bowed out in the opening round for the first time since 2003, while his last title at the Crucible came in 2013.

Ray Reardon's record as the oldest world champion has stood since the 45-year-old lifted the title in 1978 and, although O'Sullivan will be 44 in December, Doherty is wary about writing off his chances of equalling Stephen Hendry's record seven titles just yet.

"I think he could come back and win next year and then he'd only have to win one in the next few, so it's certainly still possible," 1997 world champion Doherty told Press Association Sport.

"I wouldn't disregard it completely because anything is possible with Ronnie, he's such a mercurial talent that any time he shows up he can win any tournament. I thought this was his best chance but it wasn't to be.

"It's a blemish on a fantastic season, but he wants to win seven world championships to be regarded as the undisputed greatest ever. This was a great chance for him and he's blown it so he will go back and lick his wounds. He's going to have a few other chances, but he's running out of time.

"He's come into this tournament in great form, he's had a wonderful season, played sublime snooker in the tournaments that he has played in and he's come here with a completely different attitude that I haven't seen for quite some time."

Doherty believes O'Sullivan was guilty of taking "liberties" in the opening session against qualifier Cahill, who took advantage to lead 5-4 and then held his nerve in the second session after O'Sullivan drew level at 8-8.

"Maybe he's got major health problems and that's his number one priority to be okay, but if you play like that against any opponent then you're asking for trouble and I think it caught up with him in the second session," Doherty added.

"David Gilbert played with a virus, Stephen Hendry won this tournament with a broken arm, so I don't care whether you've got a virus or not.

"If your head is not right and if you don't play the right shots, if you take liberties against any player, you're going to come unstuck and that's what happened."