World No 1 Novak Djokovic was beaten by Tomas Machac 6-4 0-6 6-1 in the Geneva Open semifinals on Friday as the Czech reached his first ATP Tour final.
Machac was down 1-4 before eventually taking the opening set as Djokovic requested a medical timeout at the end of it.
The Serb improved to fight back in the second set but faded again in the decider, allowing Machac earn his second Top 10 win.
"I have no reaction right now, I just fought for every ball," Machac said. "When you play against Novak you just hope. You just try to play your best and see what it looks like.
"I am looking forward to playing in a final for the first time."
Djokovic, who turned 37 on Wednesday, is yet to reach a final this season.
The 24-time Grand Slam winner requested the Geneva wildcard after he was hammered in Rome in the last 32 by little-known journeyman Alejandro Tabilo.
Two days earlier a freak accident saw Djokovic hit on the head by a fan's water bottle while signing autographs.
He will start the defence of his French Open title against France's Pierre-Hugues Herbert in the first round on Sunday.
Machac, who was facing a Top 10 player on clay for the first time, became the first Czech to make the final in Geneva since Tomas Smid in 1987.
He will play two-time champion Casper Ruud or Italian Flavio Cobolli for the title on Saturday.