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 will take on Jack Sock in Sunday's final, after the American beat French wildcard Julien Benneteau 7-5, 6-2 to move within one win of a Tour Finals spot. Krajinovic dug deep to come through a deciding-set tie-break against the big-serving Isner, who struck 31 aces.

He 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 Sock is the only man who can deny Carreno Busta. "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.

Krajinovic looked to be the fitter player, 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.

Sock, 25, saw off a spirited Benneteau with little trouble to deny the Paris crowd a first home finalist since Jo-Wilfried Tsonga six years ago. A topsy-turvy opening set, which saw the players share four breaks of serve in the first six games, took the crucial turn when Sock broke in game 11.

Sock, who started the week 24th in the 'Race to London' standings, had found his rhythm on serve and forged into a one-set lead. The 83rd-ranked Benneteau, who delighted the home fans with a shock win over third seed Marin Cilic in the last eight on Friday, was struggling against Sock's powerful forehand and fell a break down in game five of the second set.

The world number 22 was by far the stronger player and completed victory with a run of four straight games to close in on London.