The 23-year-old from Blackpool – who had to win three qualifying matches simply to make his Crucible debut – caused one of sport’s greatest upsets by ousting five-time champion O’Sullivan 10-8 in their first-round game.
Cahill does not even have a world ranking having fallen off snooker’s professional Tour in 2017, but held his nerve to send the in-form world No 1 – who has won five tournaments this season – crashing out.
Leading 5-4 overnight, Cahill extended his advantage to 8-5 before O’Sullivan rallied to level at 8-8.
But Cahill, who has won back his two-year Tour card for next season, held his nerve to secure a second-round meeting with Scotland’s Stephen Maguire.
“I thought I was going to fall over at one point,” reflected Cahill.
“I was a bit nervous when I went out there, but after that I settled really well.
“I felt like I was playing another tournament, which I think was a big part of it.
“He started coming back at me, but that is what you expect from Ronnie. I made a couple of good breaks to get back into a flow.
“He was under a lot of pressure himself. He didn’t want to lose to me and I’m just glad I managed to hold myself together and win at the end there.
“You’ve got to believe you can beat anyone. I’ve got a lot of respect for Ronnie, he is such a great player and is my idol really.
“But there’s only so much respect you can have for these people because at the end of the day you’ve got to go out there and beat them. If I had too much respect then I probably wouldn’t have cleared up at the end,” said Cahill, who beat then world No 1 Mark Selby in the UK Championship at York in December.
Resuming yesterday O’Sullivan quickly levelled at 5-5 after an impressive 97 break, failing to clinch an opening century after missing the final red.
He looked set to take the lead in the next frame, rattling off 48, before allowing Cahill in, and he cleared with 84 to edge back in front.
O’Sullivan was struggling, as Cahill eked out the next two frames to lead 8-5.
But the five-time world champion scrambled back as breaks of 104 and 89 helped to restore parity once again at 8-8.
Cahill opened with 62 in frame 17, but when he missed an easy red to the corner O’Sullivan looked set to pinch the frame. But on 53 he ran out of position on the pink and the Blackpool potter cleared the final two balls.
When O’Sullivan suffered a piece of misfortune – inadvertently knocking in a red after potting the blue on a break of 42 – the finishing line was in sight for Cahill, a 52 clearance prompting fist-pumping celebrations.
It is the first time O’Sullivan, who has dominated this season, winning the 2018 Shanghai Masters, 2018 Champion of Champions, 2018 UK Championship, 2019 Players Championship and the 2019 Tour Championship, has fallen at the first hurdle in Sheffield since defeat to Marco Fu in 2003.
O’Sullivan claimed he had been “under the weather” heading into the tournament, but refused to make excuses for his shock defeat.
“I will look back and think that it has been a very successful season,” he said. “I treat every tournament I play in like the World Championship. This one just wasn’t meant to be.
“I thought he did well, especially in the last couple of frames. He potted a good pink and black. That clearance at the end was a very good clearance. He held himself together well playing at the Crucible.
“I just felt a little bit under the weather. My legs are like lead and my arms feel really heavy.
“I’m not making excuses, I just felt absolutely exhausted to be honest.
“I tried to do as much as I could, to hang in there and see if I could manage to somehow get through the match, and after a few days I might have felt better. But it wasn’t meant to be.”
David Gilbert will face defending champion Mark Williams in the second round after holding off a fightback from qualifier Joe Perry to win 10-7.