Brilliant to beaten - Final moments of O'Sullivan's shock exit defeat to amateur CahillBetfred World Snooker ChampionshipVenue: Crucible Theatre, Sheffield Dates: 20 April-6 MayCoverage: Watch live on BBC One, BBC Two, Red Button, iPlayer, Connected TV, BBC Sport website and BBC Sport app.
Amateur James Cahill pulled off the biggest shock in Crucible history as he completed an astonishing 10-8 first-round win over five-time champion Ronnie O'Sullivan.
O'Sullivan trailed 5-4 overnight and the increasingly rattled world number one slipped 8-5 behind, missing countless simple chances against the 23-year-old qualifier.
A jaded-looking O'Sullivan then somehow found some form, scoring breaks of 104 and 89 to level.
But Cahill, who does not even have a world ranking, showed remarkable composure under pressure to get over the line.
"I could barely stand up at the end," the Blackpool potter told BBC Sport. "I am not really sure what to say.
"I scored a good pressure 70 to go 6-5 up and after that I felt like he was the one under pressure. He didn't want to lose to me.
"I have always believed in myself and that I can beat anyone on my day. I want to show what I can do now."
O'Sullivan was set to go 9-8 ahead but missed a relatively simple final pink to allow Cahill, the first-ever amateur to make it through to the Crucible, the chance to clear up.
And he made sure of his place in round two, when he will face Scotland's Stephen Maguire, with a fine clearance of 53 in the final frame.
Cahill lost his professional status in 2017, but has won back his two-year tour card from next season after a strong year playing as a top-up player on tour.
He has now backed up his impressive first-session performance in the pressure-filled arena against a man considered the greatest player of all time.
The other match in Wednesday's morning session saw 2015 world champion Stuart Bingham open up a commanding 8-1 lead over 2006 winner Graeme Dott.
Debutant James Cahill punched the air after his first-round victoryO'Sullivan not at 100%
Pre-tournament O'Sullivan had talked about the need to win this year's event if he is to stand a chance of equalling Stephen Hendry's seven world titles.
But he told the BBC that his "limbs felt heavy" and he "had no energy".
"I felt horrendous. I was struggling to stay awake," the 43-year-old said.
"I haven't felt great for a few weeks and I have not slept brilliantly the past couple of nights.
"You have to come here physically and mentally well. If you are not 100% it will make it harder. I tried to hang in there and get through and have a few days off.
"He did well. He held himself together. It's been a very successful season for me, but it wasn't meant to be."
Defeat means the 36-time ranking event winner has not reached the World Championship quarter-finals since 2014 and this is just the fourth time he has lost in the first round at snooker's showpiece event in his 27 visits.
O'Sullivan, who has won three ranking titles in 2018-19, often looked dejected in his chairAnalysis
Six-time world champion Steve Davis on BBC Two
What a fantastic performance by James Cahill. The way he cleared those balls up under significant pressure - because they were there to be taken - sometimes that makes it an even harder job.
Knowing what was probably going through his mind about how big a deal it would be, to still hold himself together to pot that last red, and then clear what was a normal set of colours under that pressure, absolutely fantastic and he must be absolutely delighted.
For Ronnie O'Sullivan that first session was a tough one. He looked yesterday like he didn't want to be there and I don't know the reasons for that, but he played like he didn't want to be there.
He was bailing out in some shots just by attacking all the time. He's an attacking player but when you are attacking so much it almost goes the opposite way. It's sort of like you're hoping to be knocked out, in a strange way.
We always think Ronnie O'Sullivan is super human and doesn't suffer with nerves, but if you are going to suffer with nerves the Crucible is where it is going to happen. The enormity of the World Championships weighs heavy on him.
