Borna Coric upset Rafael Nadal 62 76 to reach the semifinals of the Swiss Indoors in Basel.
Coric next plays David Goffin, who beat beat Milos Raonic 6-7 (3), 6-3, 6-4.
Nadal, who announced after the match he will have season-ending appendicitis surgery and was playing just his third tournament since Wimbledon due to a wrist injury, looked nowhere near his best while Coric was in sparkling form.
"It was difficult in the whole match, in the whole tournament," Nadal said. "Every day I felt more tired, I tried to practice, to get fit but it was not possible. The past four, five months have been very tough for me. It's been difficult to play and compete."
"It's an unbelievable feeling," Coric said. "I'm so proud of myself and I hope I can play the same game tomorrow.
"There was pressure but I didn't have anything to lose, because if I lose I lost to the No. 1 in the world. I never played against him so I didn't know what would happen, whether he would kill me or not."