Paris - Serbian qualifier Filip Krajinovic stunned John Isner on Saturday with a thrilling 6-4, 6-7 (2/7), 7-6 (7/5) victory to reach the Paris Masters final and end the American's World Tour Finals bid.
The world number 77, who will play in his maiden ATP final on Sunday, dug deep to come through a deciding-set tie-break against the big-serving Isner, who struck 31 aces.
Krajinovic is the lowest-ranked player to reach a Masters final since then-world number 191 Andrei Pavel in the French capital in 2003.
The 25-year-old reached the last four after world number one Rafael Nadal withdrew from the tournament before their quarter-final on Friday with a right knee injury.
"This is the best day of my life," an emotional Krajinovic told the crowd.
Isner, 32, needed to win the tournament to grab the last place for the year-ending eight-man event in London from Spaniard Pablo Carreno Busta.
Now his compatriot Jack Sock is the only man who can deny Carreno Busta, and he takes on French wildcard Julien Benneteau in the second semi-final later on Saturday.
"Very disappointing. I had the chance to do something I'd never done before," admitted Isner, who was looking for a first Tour Finals appearance.
Krajinovic saved three break points in his opening service game and grew into the contest from then on, breaking Isner in game five.
It was only the third time the world number 14 had dropped his serve in the tournament, and Krajinovic closed out the first set with a confident hold to love.
The Serb had only won two matches at Grand Slam or Masters tournaments in his career before this week, but he has claimed five second-tier Challenger titles in 2017 to return to the world's top 100 after struggling with injuries in recent years.
Isner, who like Sock was looking to become the first American to win a Masters title since former world number one Andy Roddick in Miami seven years ago, solidified his serve at the start of the second set.
Krajinovic looked to be the fitter player, though, having had a day off due to Nadal's withdrawal, while Isner had been taken to a deciding set in all of his three previous matches.
But the giant American, runner-up in Paris to Andy Murray last year, found a second wind in a second-set tie-break, reeling off the first five points in a row.
He closed it out with two massive aces to take his tally for the match to 16.
Krajinovic saved a rare break point to edge ahead in the third set, but Isner continued to rattle through his service games, including one that featured four thunderous aces.
Isner grabbed an early 3-0 advantage in the deciding breaker, but Krajinovic showed great resolve to take five of the next six points.
One final ace levelled for Isner, but Krajinovic brought up a match point on his own serve with a magnificent backhand winner and clinched victory by crunching a forehand down the line.