An exhausted Ronnie O'Sullivan will spend the rest of the World Championship as an interested spectator after suffering a shock first-round defeat to amateur James Cahill at the Crucible.

O'Sullivan was the favourite to claim 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.

"I just felt a little bit under the weather, my legs are like lead and my arms feel really heavy," O'Sullivan said after losing in the opening round for the first time since 2003.

"I'm not making excuses, I just felt absolutely exhausted to be honest with you. I just tried to do as much as I could, just tried to hang in there and manage to somehow get through the match and might have felt a bit better after a few days off, but it wasn't meant to be.

"If I feel better Friday I'll be really disappointed, if I still don't feel too great it probably wasn't the worst thing that could have happened. It's over, it's done and now I can disappear and have a holiday and just enjoy the rest of the tournament.

"I think it's been a very successful season. Every tournament I play in I treat it like a World Championship and this one just wasn't meant to be, but there's been plenty of other good tournaments.

"It's just life, it's not the end of the world. I tried to give it my best, I tried my hardest and it wasn't good enough."

O'Sullivan recovered from 8-5 down to get back on level terms and looked set to move 9-8 in front before missing a vital pink, while he was unfortunate to knock in a red when splitting the pack from the blue in the next frame and Cahill held his nerve to compile a match-winning break of 53.

"I thought he did well, especially the last couple of frames," O'Sullivan added. "He potted a good pink and black and that clearance in the end was very good - he held himself together well on his debut in the Crucible."

O'Sullivan said he had not been feeling 100 per cent since winning the Tour Championship in Llandudno and also revealed he was continuing to suffer from insomnia.

"I've had it for quite a few years now and the problem is after three or four years you get used to having two or three hours sleep," the 43-year-old added.

"The last two years I've been sleeping a bit better and then when you have a spell where you don't sleep well you really feel it. I would rather have full-on insomnia and your body learns to adapt to anything."

Ronnie O'Sullivan congratulates James Cahill (R)

O'Sullivan's defeat means he has not reached the semi-finals at the Crucible since finishing runner-up in 2014 and the prospect of matching Stephen Hendry's record of seven world titles appears increasingly remote.

"Some people are driven by records, some people are not," O'Sullivan added. "If they are then they'll probably want to go away and work at it and come back again.

"Some people are not and just take it event by event and just have more short-term goals I suppose." 

Cahill came through three rounds of qualifying to make his Crucible debut, beating Irishman Michael Judge to become the first amateur to play in the first round proper.

The 23-year-old from Blackpool said: "I'm absolutely over the moon with that. I'm glad to hold myself together against the best player in the world on my Crucible debut, it doesn't get any better than that.

"At the end when he starts coming back that's what you expect against Ronnie, but I did a couple of good breaks to sneak frames back when it felt like he was just about to get on a bit of a flow and he's under a lot of pressure himself, he doesn't want to lose to me.

"You've got to believe you can beat anyone. I've got so much respect for Ronnie, he's such a great player and my idol really, but there's only so much respect you can have for these people because at the end of the day you have to go out and beat them.

"If you have too much respect for them I probably wouldn't have cleared up at the end there. I don't really know [how I did it]. I thought I was going to fall over at one point."

Cahill lost his status as a professional in 2017 but has regained his place on the tour from next season and is determined not to rest on his laurels after beating O'Sullivan.

"I'm here to win the tournament," added Cahill, who beat world number one Mark Selby in the opening round of the 2018 UK Championship.

"I play every tournament to win otherwise I wouldn't play. You can't come to a tournament thinking you're going to get beat... or what's the point in playing?

"I've proved that I can play. I made the most centuries out of all the professionals in the qualifiers so the amateur thing doesn't mean anything.

"I don't have to prove anything now, I've got my two-year card, I've just beaten Ronnie, the world number one and I'm going to give it my all in my next game and see what happens."