Basel - Two-time champion Juan Martin del Potro went down swinging with 14 aces in a 7-5, 6-4 quarter-final loss to Kei Nishikori at the Swiss Indoors on Friday.
Defeat in just under two hours 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 Roger Federer -- when he takes on Gilles Muller, a 6-7 (4/7), 6-4, 7-6 (7/3) winner over Argentine Federico Delbonis.
Top seed Stan Wawrinka was playing qualifier Mischa Zverev in a later quarter-final while World Tour Finals hopeful Marin Cilic was bidding to reach the semi-finals as he played Spain's Marcel Granollers.
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."