Alexander Zverev has beaten world number one Novak Djokovic 6-4 6-3 to win the ATP Finals in London, the biggest title of the German's fledgling career.
The 21-year-old broke serve once in the opening set and three times in the second to pull off a shock, denying Serbian Djokovic a record-equalling sixth title at the ATP's blue-riband event.
Djokovic was a firm favourite having looked unbeatable all week, not dropping a set or even a service game, but 24 hours after being booed following a semi-final win over Roger Federer, Zverev had the crowd drooling over an audacious display.
Zverev is the first German to win the title since Boris Becker in Frankfurt in 1995 and the youngest since Djokovic won at the same age in Shanghai in 2008.
Djokovic had beaten Zverev comfortably in the round-robin phase on Wednesday, but he looked edgy.
At 4-4 in the opener a couple of poor Djokovic forehands gave Zverev the chance to serve for the set and he sealed it thanks to three consecutive aces.
An increasingly ragged Djokovic then dropped his opening two service games of the second set, either side of breaking Zverev for the first time.
With victory looming world number five, Zverev showed no sign of nerves and clinched the title when he sent a backhand winner past a stunned Djomovic.
He collapsed onto his back before Djokovic sportingly walked around to the other side to congratulate him.
Meanwhile Mike Bryan claimed his fifth ATP Finals doubles title, but the first without his twin brother Bob, as he and Jack Sock beat French pair Pierre-Hugues Herbert and Nicolas Mahut in their final.
The American duo won 5-7 6-1 13-11 as they added the season-ending crown to the Wimbledon and U.S. Open titles.
Bryan only paired up with Sock in June because brother Bob, with whom he has won 16 Grand Slam titles and Olympic gold, is currently recovering from a hip injury.
The 40-year-old Bryan becomes the oldest champion at the tournament.
Roland Garros champions Herbert and Mahut had been bidding to become the first French pair to win the ATP Finals title for 13 years. They will be in action at next week's Davis Cup final.