Serbian qualifier Filip Krajinovic has knocked John Isner out of contention for the ATP year-end event as he upset the American 6-4 6-7 (7-2) 7-6 (7-5) to reach the final of the Paris Masters.
Krajinovic will face another American Jack Sock in Sunday's decider.
The loss ended Isner's hopes of claiming the eighth and final spot at the ATP Finals, which begin in London next Sunday.
Sock, the 16th seed, needs a title this weekend to claim a London place after breaking French hearts in the semis with his 7-5 6-2 win over Julien Benneteau, who retires next season.
Sock moved through in 78 minutes as he broke five times.
Americans have had little recent form at Bercy, with Andre Agassi the last to play a final at the indoor event in 1999 while Andy Roddick earned the last Masters trophy for the country seven years ago in Miami.
Despite a 31-ace performance, Isner was caught out in the final-set tiebreaker as the 77th-ranked Krajinovic returned a big serve with interest, earning a match point against the biggest man on the tour.
Krajinovic didn't let his opportunity go to waste as he finished the job with an overhead winner to the corner, well out of Isner's reach.
"The emotions are amazing, big, it was a tough mental match," the winner said. "He was serving really well, he was playing aggressive.
"When I served for the match, my hand was shaking. It was tough to control the emotions, but I knew I had to stay aggressive all the way to the end. I did it, and it's the best day in my life.
"But the tournament is not over yet. I'll prepare for tomorrow and then we'll see.
"It's very, very disappointing," Isner said.
"I had an opportunity here to do some things I've never done before, finish in something like in the top 10.
"It's never easy to lose a very important match - it stinks."
Krajinovic was playing only the second ATP-level semi of his career and his first since 2010. He received a walkover in the quarters when Rafael Nadal quit before their match with a knee problem and returned to Barcelona for medical attention.
