Swiss Indoors: Top seed Wawrinka shown the door by German qualifier Mischa Zverev

German qualifier Mischa Zverev ousted US Open champion Stan Wawrinka to advance to Swiss Indoors semi-finals.

Top seed Wawrinka lost to Zverev 2-6, 7-5, 1-6. Zverev will now face fourth seed Marin Cilic in the semis who beat Marcel Granollers 6-3 6-3. 

Wawrinka has been playing the event for 13 out of 14 years of the tournament and has only reached semis once in 2011 against Federer in 2011.

History was not so kind this time either. Zverev broke Wawrinka for 2-5 in the first set. The Swiss managed to win the second set but went down a double break in the last set on his way to defeat in just under two hours.

"Zverev almost beat Novak Djokovic two weeks ago in Shanghai," Wawrinka said according to an AFP report.

"He is an offensive player who likes to rely on the speed of the ball from the opponent."

In the other quarter-final clash, two-time champion Juan Martin del Potro was defeated by Kei Nishikori for 7-5, 6-4. This finished the ATP season for del Potro who had a fantastic run in 2016.

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.

Nishikori will play Gilles Muller who defeated Argentine Federico Delbonis.

"This was my best match of the week. 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."