Novak Djokovic suffered only his fourth defeat of the year as he lost to Japan's Kei Nishikori in the semi-finals of the Swiss Indoors in Basel.
The world number one, who was nursing a shoulder injury, lost 2-6 7-6 (7-4) 6-0 to the world number 32.
Wildcard entry Nishikori will now meet Roger Federer in the final after the Swiss beat Stanislas Wawrinka.
The Japanese player is only the second man, after Federer, to beat Djokovic in a completed match this year.
Two of Djokovic's previous four defeats had come as the result of retirements.
The top seed received treatment for his shoulder problem early in the match and needed another injury timeout at the start of the final set.
After the match he admitted his shoulder had been troubling him all week, but said he still expected to play in the Paris Masters, starting on Monday.
"I don't think I'll be able to train for the next few days, but I think I'll go straight to Paris," he said.
And the reigning Australian, Wimbledon and US Open champion paid tribute to his opponent, who has just one career title - on hard courts at Delray Beach, Florida, in 2008 - to his name.
"He was getting impossible balls back and really making me play every shot," said Djokovic. "He was better and I didn't use the opportunities I had. I don't think I should speak about that third set."
The Serbian was playing in his first tournament following six weeks out with a back problem.
The 32nd-ranked Nishikori said: "It's unbelievable. In the second set I started playing well and getting a rhythm."
In the other semi-final, Federer beat compatriot Stanislas Wawrinka 7-6 (7-5) 6-2. The world number four is playing in his first tournament since the US Open and is bidding to win only his second title of the season following Doha in January.
Federer hit with Nishikori only hours before the Japanese player beat Djokovic, and insists he will treat his opponent with respect in Sunday's final.
"He (Nishikori) has got great shots and has been on fire for the past month. He's got a quick arm as well. I've got to go out there and take advantage of my chances."