The 21-year-old German, who had been booed off court at the O2 Arena following his straight sets semi-final win over Roger Federer, defeated his Serbian opponent 6-4, 6-3.

"It's the biggest title I have won," said the amiable German, who made a point of thanking the "knowledgeable crowd" while referring to "our moment" in his previous match.

On that occasion an apparently pro-Federer crowd reacted after Zverev had shouted and stopped playing during the second set tiebreak when a ball boy accidentally dropped a ball midpoint - which by the game's rules means the point must be replayed.

It meant Zverev reverted to having a first serve, from which he produced an ace and after which went on to win.

ATP Finals tournament director Adam Hogg criticised the section of the crowd who booed, labelling the reaction as "astonishingly disappointing".

Zverev had lost to Djokovic in straight sets in their group match, and the latter player looked odds-on to achieve his ambition of winning a sixth ATP Finals title having reached the final without dropping a set.

But Zverev's potent combination of talent and resilience wore Djokovic down and the 31-year-old looked a little lacklustre by the close.

Thus the young German, who has been receiving coaching input from eight-times Grand Slam winner Ivan Lendl since August, has beaten two of the game's legendary champions in the space of 24 hours.

He is the youngest champion in the ATP Finals since Djokovic himself 10 years ago.

Zverev's fellow countryman and multiple Grand Slam winner Boris Becker said on BBC TV: "A German with a sense of humour who can talk and laugh at himself - a star has arrived."