Karen Khachanov upset a tired-looking Novak Djokovic 7-5, 6-4 to win the Paris Masters title and deprive Djokovic the chance on Sunday of matching Rafael Nadal's record of 33 Masters titles.
Djokovic, a record four-time champion at the indoor event, looked out of energy after an epic three-hour semifinal win against Roger Federer on Saturday.
Although Djokovic broke in the fourth game to move 3-1 up and then led 30-0 on serve, the unseeded Khachanov broke him straight back and the momentum abruptly shifted away from Djokovic.
The Serb seemed agitated at times and twice turned to his box to remonstrate about an unspecified issue during the first set.
Khachanov broke for 6-5 when he hit a powerful shot down the line which Djokovic could only scoop back into the net. The unseeded Russian won the first set with a big first serve which Djokovic could not return, stretching out his racket in vain as the fizzing ball clipped the frame.
Djokovic struggled to handle Khachanov's brutal two-handed backhands from the baseline, which often landed near his ankles, and dropped his serve again to trail 2-1 in the second set. He had to save three more break points in the seventh game to hold for 4-3 down.
Khachanov secured victory on his first match point when Djokovic chopped a backhand return wide. The imposing Russian thrust both his arms in the air and, moments later, knelt down to kiss the court.
Djokovic will return to No. 1 in the rankings for the first time in two years on Monday, but he will be disappointed at missing out on a 73rd career title, having withstood the best of Federer on Saturday.
The 22-year-old Khachanov, ranked 18th, added this title to the Kremlin Cup in Moscow last month for his third title of the year and fourth overall.