The world No 1 saw his fightback from an overnight 5-4 deficit fall short as the five-time world champion suffered a 10-8 defeat against Cahill, who at 23 years old became the first amateur to reach the Crucible and made history even before a ball was struck.
O’Sullivan looked out of sorts on both Monday and Tuesday as he struggled for consistency and appeared lethargic around the table and in his chair, and confirmed afterwards that his energy levels were so low that he was having to fight falling asleep when not playing.
“Yeah I’m alright, all my limbs just feel really heavy – my legs and arms – I feel absolutely shattered, just drained really,” O’Sullivan told the BBC. “I had no energy, struggling to stay awake. I haven’t got any visual stuff, I just feel absolutely horrendous to be honest.
“You just can’t choose. You have to come here physically and mentally feeling good, and mentally I was feeling up for it. I’ve had a good season. I didn’t expect to do well but you just come here and try to do your best and if you’re physically not 100 per cent then it’s going to make it even harder. I just tried to hang in there and do as much as I could and just see if I could get through this match and have a few days off and just try to feel a little bit better.”
Cahill took a 5-4 lead into Tuesday’s early session but immediately saw his advantage whittled away as O’Sullivan – sporting a new shaved haircut since Monday’s action – opened up with a break of 97 to make it 5-5 in the race to 10 frames.
But Cahill opened up a two-frame advantage after O’Sullivan wasted chances to surge ahead, and that quickly became three as O’Sullivan’s error-strewn display proved clear evidence that he was off his game and struggling with the first-round encounter.
But O’Sullivan started to fight back in winning consecutive frames and produced a break of 89, which only ended when he called a foul on himself in potting a red, to get back on level terms at 8-8.
O'Sullivan looked on course to set up a final-frame decider only to inadvertently pot a red as he broke open the pack from the blue. That gifted Cahill the chance to claim the biggest win of his career and he held his nerve superbly with a break of 53 to win 10-8.
“Well there’s two ways you can look at it,” added O’Sullivan. “You can look at it in a way that he’s come here and played brilliantly and fair play to him, or you can also look at it as I left him pretty easy chances that allowed him to gain confidence. So there’s two ways you can look at it but whatever way you do look at it you have to give credit to him that he finally got over the line.
“I played a bad shot on the blue (in the 17th frame at 8-8) and left myself a horrible angle really and I tried to pot it with side and missed it and he potted the pink and black.
“You can look at it other ways, you can say the players that have played more probably deserve to have a better run in this tournament or you can just say I might have played the right amount and it just wasn’t meant to be you know, just one of those things.”
Cahill goes through to the second round where he will face 15th seed Stephen Maguire, who defeated Tian Pengfei 10-9 in another tense first-round encounter.