Reigning title-holder O'Sullivan resumed 10-6 ahead only to lose seven frames in a row as Belgium's Brecel, ranked tenth in the world, recorded a 13-10 victory at Sheffield's Crucible Theatre.
"Before the tournament I was out partying and staying up late until 6 or 7am, playing FIFA with my friends, having drinks and not practising," Brecel told the BBC.
"Even after I beat Mark Williams I got home at 7am by car, and that same day we went out again until five or six in the morning," the world number 10 added.
"I was drunk as hell. The next day I had to drive back up again, so it's a totally different preparation."
The 47-year-old O'Sullivan, bidding to win a modern-day record eighth world title, failed to make a single break over 30 as Brecel rallied to set up a last-four clash with either Scotland's Anthony McGill or China's Si Jiahui.
'Incredible'
O'Sullivan said Brecel had played an "unbelievable" match, with the Englishman adding: "His cue action, he gets through the ball so well. It's incredible.
The whip he gets on the white, the top spin, the thud he hits the ball with.
"He's such a dynamic player, probably the most talented snooker player I've ever seen.
"I'd love to see him go and win it because that's how snooker should be played. He's a phenomenal talent and player."
Few might have given much for Brecel's chances of overturning a four-frame deficit, but the 28-year-old winner of two major ranking tournaments insisted he'd always had confidence in his ability.
"It was a great feeling to play like that. I know that if the balls go well for me I can easily win seven frames in a row even against Ronnie, so I am not surprised," Brecel said.
Brecel capitalised on O'Sullivan somehow missing a black off its spot with a clearance of 112 before another error by 'The Rocket' in the next frame was punished with a break of 64.
O'Sullivan's lead was eventually reduced to 10-9 before Brecel drew level with a break of 72.
Any thoughts the mid-session interval might disrupt Brecel's rhythm, evaporated as he made breaks of 61, 78 and 63 to complete a remarkable comeback.
"If it was a boxing match they would have stopped it very early on," said O'Sullivan.
"I was just pinching frames and hanging on."
"There's only so much hanging on you can do at the Crucible. It catches up with you at some point, and someone will eventually put you away."
Northern Ireland's Mark Allen reached the semi-finals for just the second time in his career with a gruelling 13-10 win over qualifier Jak Jones where he made just one break over 70.
