Basel - Top seed and US Open champion Stan Wawrinka slumped to a shock 6-2, 5-7, 6-1 defeat to German qualifier Mischa Zverev on Friday, limping out of the Swiss Indoors event in the quarter-finals.
Zverev, 29, and ranked 72 in the world, will now face Croatian fourth seed Marin Cilic, who beat Spain's Marcel Granollers 6-3, 6-3, for a place in the final.
Wawrinka has a poor history at the Swiss tournament played in Roger Federer's hometown, despite playing the event for 13 of the last 14 years.
He ended a run of four straight first-round defeats with a pair of wins this week having made his only real impression in 2011 with a semi-final against Federer.
Wawrinka was broken for 2-5 in the first set, got the tie back level by winning the second before going down a double break in the decider on his way to defeat in just under two hours.
"Zverev almost beat Novak Djokovic two weeks ago in Shanghai," Wawrinka said.
"He is an offensive player who likes to rely on the speed of the ball from the opponent."
Two-time champion Juan Martin del Potro hit 14 aces but still went down to a 7-5, 6-4 loss to Kei Nishikori.
Defeat wrapped up the ATP season for the Argentine, who will skip next week's Paris Masters to return home to train for the Davis Cup final against Croatia.
Nishikori will Saturday play to reach his second Basel final - he lost the 2011 title match to Federer - when he takes on Gilles Muller, a 6-7 (4/7), 6-4, 7-6 (7/3) winner over Argentine Federico Delbonis.
Wild card Del Potro, whose ranking of 42 is 1,000 places better than in February when he returned after almost a year of wrist problems and surgery, lost for the first time to third seed Nishikori.
The pair last played four years ago at the London Olympics.
Nishikori dealt out only seven aces, but fired his last one to earn a pair of match points -- one was enough as he put away a volley winner at the net.
"This was my best match of the week," said Nishikori as he ended a 12-match win streak for Del Potro at the Basel St Jakobshalle venue where the South American claimed the 2012 and 2013 titles over Federer.
"It was a tough match. I had so many chances, so many break points that I could not convert," said Nishikori, who fended off all eight break points against his serve.
"I played good tactics today, everything was working well."
Del Potro was pleased with his week which came after he lifted his first title in nearly three years at Stockholm last Sunday.
"I've been trying to fix my problems for the past two years, and I didn't expect to win a title at this moment in my career. It's still a motivation for looking forward to the future.
"I felt exhausted during the whole match, I couldn't fight as I wanted. I had chances at the start of the first set, but Kei played very smart.
"I've played so many matches, it's too much for me. But I'm finishing the season well, I'm so glad for that."