The nine-time French Open champion and 14-time Grand Slam title winner confirmed he will have his appendix removed on November 3rd.
Given the circumstances the 28-year-old Spaniard said there was no logic in him competing at either next week's Paris Masters or the eight-man World Tour Finals in London from November 9th.
"This is the day to say goodbye for the season," said Nadal.
"It's the day to say I had a good first six months this year," he said.
"But it was hard after that with back and (three-month summer) wrist problems and now the appendix. I'm not going to play Paris or London, I'm not competitive enough to do that.
"I need to do the surgery, I need to work on my back next week before the operation. Then I need five weeks to recover."
Top seed Roger Federer turned in another command performance, mowing down fifth-seeded Bulgarian Grigor Dimitrov as the Swiss continued his bid for a sixth home trophy.
Federer posted a 7-6 (7/4), 6-2 win to set up a semifinal with the biggest server in the game, Ivo Karlovic after the eighth seed beat German Benjamin Becker 6-4, 6-4.
"It's like a penalty shootout playing him," said Federer of the leading ace-maker on the ATP this season with more than 1,000.
"You just pick a side on his serve and hope you are right. There's really no strategy."
Federer's win was his 65th of the season and lifted his record at the tournament to 54-9.
Coric's win in just under one and three-quarter hours announced the 17-year-old Croatian as a name for the future.
The wildcard raced away with three straight breaks of Nadal, taking a 5-0 lead in the opening set.
"This is a huge surprise," said the world number 124.
"I couldn't believe I got to 5-0 in the first set against Nadal. I was scared he would start a comeback. But I stayed very calm and I played well."
David Goffin, seeded seventh, knocked off London year-end finals contender Milos Raonic, with the fourth seed from Canada exiting 6-7 (3/7), 6-3, 6-4.
Goffin is having his best season, with a pair of ATP titles and four more at Challenger events.
"It's one of my nicest victories, if not the nicest," said Goffin.
"It's really pleasing," he said.
"I can't really comprehend what I've achieved since Wimbledon: more than 40 victories, two titles . . . I think I'll only realize what I've achieved when I'm on holiday."
Goffin and Coric face off in Saturday's other semifinal.