London - Alexander Zverev emphatically announced his arrival at the top of the men's game by overpowering Novak Djokovic 6-4, 6-3 to win his first ATP Finals title on Sunday.
The 21-year-old German, making his second appearance at the end-of-season showpiece, outlasted six-time champion Roger Federer in the semi-finals and repeated the feat against the world number one.
Djokovic had not been broken once in the tournament coming into the match, winning all 36 service games and only facing two break points but Zverev wrecked his numbers, breaking four times in the match.
The German, coached by Ivan Lendl, faltered just once when Djokovic broke him early in the second set but that was the only blemish on an extraordinary performance of power and skill.
"I really can't describe it," said Zverev.
"It is the biggest title I have ever won. Firstly, I want to congratulate Novak and we may never have seen the tennis he has played in the last few months before. He barely lost a match but thankfully he did to me.
"We (Djokovic and Zverev) had so many talks, not only about tennis but all different types of subjects - I won't mention what - but you are a sharing person and you have shared some titles with me. I appreciate you letting me win one today."
The Serbian top seed, 31, came into the match seeking to equal Federer's record of six ATP Finals wins but was immediately aware he had a fight on his hands at London's O2 Arena.
Djokovic beat Zverev 6-4, 6-1 in their round-robin match on Wednesday but it was a different story in front of a packed and vociferous crowd on Sunday.
With both players going toe-to-toe from the back of the court, the first set went with serve until the ninth game, when Djokovic dumped a forehand into the net to concede the break.
Roared on by the crowd, Zverev hit three aces on his way to sealing the first set 6-4.
Still on a high, the third seed broke an out-of-sorts Djokovic immediately at the start of the second set to leave the top seed with a mountain to climb.
Showing nerves for the first time, he double-faulted twice in his next service game as Djokovic got back on level terms but the German broke again to lead 2-1 as the Serb's error count mounted.
Zverev broke Djokovic for the third time in the second set in the ninth game, producing a stunning backhand down the line at full stretch to win the championship.
The young German has often been talked about as the leader of the next generation but despite having three Masters titles under his belt in his short career, he has a poor record at the Grand Slams.
Djokovic slipped to an unaccustomed low of 22nd in the world earlier this year but has lost just three matches since the beginning of Wimbledon as he charged back up the rankings.
"Sascha (Zverev) definitely played much better than he did in the group stage," he said.
"He deserved to win. He's still quite young but he's had an amazing career so far and I wish him all the best for his future.
"Speaking of being young, hopefully I'll play many more years and I'll see you guys here in years to come."
Results on the final day of the ATP Finals at London's O2 Arena on Sunday (x denotes seeding):
Alexander Zverev (GER x3) bt Novak Djokovic (SRB x1) 6-4, 6-3
Mike Bryan/Jack Sock (USA/USA x5) bt Pierre-Hugues Herbert/Nicolas Mahut (FRA/FRA x8) 5-7, 6-1, 13-11