An inspired Alexander Zverev beat world number one Novak Djokovic 6-4 6-3 to win the ATP Finals on Sunday -- 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 victory in a packed 02 Arena -- 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 Djokovic.

He collapsed onto his back before Djokovic sportingly walked around to the other side to congratulate him.

Zverev described his ATP Finals title as "astonishing" on Sunday after beating Roger Federer and Novak Djokovic in consecutive days without losing a set. Not only did he become the youngest winner of the title since Djokovic 10 years ago, and the first German to triumph since Boris Becker in 1995, Zverev also became only the fourth player to beat Federer and Djokovic back-to-back in the semi-final and final of the same tournament. 

Rafael Nadal has managed it three times, Andy Murray once and David Nalbandian is the other.

"Obviously it's quite astonishing, winning this title, beating two such players back-to-back, Roger and Novak, in semi-finals and finals. (It) Means so much," Zverev told reporters.

"I'm incredibly happy and incredibly proud right now." 

Zverev said the loss to Djokovic had prompted a different approach, following a discussion with coach Ivan Lendl -- the eight-times Grand Slam champion who helped Britain's Murray to multiple major titles.

"He obviously analysed the match that I played with him a few days ago, told me a few things I had to do different. I was more aggressive today," Zverev, who will end the year ranked fourth in the ATP rankings, said.