Snooker great Ronnie O'Sullivan suffered a stunning defeat when he was beaten by amateur James Cahill in the first round of the World Championship on Tuesday.

In one of the biggest upsets in snooker history, Cahill beat five-time world champion O'Sullivan 10-8 at the Crucible Theatre in Sheffield.

Not since 2003 had O'Sullivan lost in the first round of the World Championship.

O'Sullivan, the current world number one, had been in fine form heading into the world championship after winning five tournaments this season, compiling his 1,000th career century en route to victory in the Players Championship.

But that did not stop Cahill establishing a 5-4 overnight lead which the 23-year-old extended to 8-5 when the match resumed Tuesday.

O'Sullivan pulled back to 8-8 with the aid of breaks of 104 and 89.

But the 'Rocket' missed a chance to take the 17th frame, leaving Cahill 9-8 in front.

O'Sullivan also had the opportunity to take the match to a final-frame decider, but the 43-year-old inadvertently potted a red when breaking the pack open from the blue.

Cahill made the most of an unexpected chance by holding his nerve with a break of 53 that saw him to a memorable win and into a second-round match against Scotland's Stephen Maguire.