Alexander Zverev stunned world number one Novak Djokovic in straight sets to win the season-ending ATP Finals and earn the biggest victory of his career.
Djokovic, 31, was the heavy favourite to win a sixth finals title but was outclassed 6-4 6-3 by the German.
Zverev, the third seed, broke Djokovic for a 5-4 lead in the first set, then served three big aces to take the set.
The 21-year-old continued to dominate in the second and sealed a memorable victory as Djokovic faltered.
“I’m unbelievably happy. Obviously it is the biggest title I have ever won,” said Zverev, who took home about £2m in prize money.
“How I played today, how I won it, for me it’s just amazing.”
He had lost to Djokovic in straight sets in the round-robin stage but, after beating Roger Federer in Saturday’s semi-finals, produced a more confident performance to stop the Serb equalling Federer’s haul of end-of-season titles.
Zverev missed his first championship point before hitting a sensational backhand down the line for victory, falling to the court in a mixture of disbelief and celebration as Djokovic jumped over the net to congratulate him.
“You definitely played much better than you did the group stage,” Djokovic told him with a laugh. “You deserved to win.”
Zverev has long been heralded as the young player most likely to carry the mantle for the men’s game once the likes of Djokovic, Federer, Rafael Nadal and Andy Murray have retired, with the 6ft 6in German’s booming serve, crunching groundstrokes and improved play around the net showing he has all the facets needed to reach the very top.
Against Djokovic he also showed mental resilience and staying power in gruelling rallies to prove he is the real deal.
A high-quality first set remained on serve in the opening eight games, both players only managing to win three receiving points up to then, before a stray forehand from Djokovic – a rare unforced error this week – brought the first break point of the match in the following game.
Djokovic landed a deep first serve but when it came back over to his side of the court could only crack a forehand into the net.
For the first time in the tournament he had lost his serve, and for the first time he looked rattled.
The true test for Zverev was to keep his nerve and serve out for the first set, a question answered by the German firing down three aces before sealing the set as Djokovic went long with a backhand after a short rally.
Zverev kept up the attack at the start of the second set, creating two break points in the opening game – one by outlasting Djokovic in a 26-shot rally – and taking the second opportunity.
Djokovic was somewhat gifted the break back as Zverev produced two double faults but any thought that might swing the momentum back to the Serb proved unfounded, Zverev breaking again to hold a lead which he would not relinquish.
Djokovic, who had won 35 of his previous 37 matches, suddenly lost the control he had shown all week and was unable to make Zverev serve out for victory as the German broke again to win in one hour and 19 minutes.
Zverev’s next challenge is to show he can perform over the five-set format at the Grand Slams.
This year he reached his first Grand Slam quarter-final at the French Open and it looks only a matter of time – especially with eight-time major champion Ivan Lendl now in his corner – that he will go deeper into the biggest tournaments.
Djokovic started the week as the hot favourite to win at the O2, following a remarkable resurgence which saw him win Wimbledon and the US Open on his way to regaining the number one ranking after being outside the top 20 in July.
Djokovic’s relentless returning has been the hallmark of his game and before the final he stressed the importance of putting the ball back into play on Zverev’s first serve.
Although he managed to do that regularly, Djokovic perhaps did not account for Zverev being able to stick with him in the long rallies.
For once Djokovic was unable to wear down his opponent and, coupled with his below-par service game, was unable to seriously threaten Zverev.
Djokovic appeared to wane mentally as the match slipped away from his grasp, although defeat here cannot take the shine off an extraordinary year.
“He had big serves. I wasn’t returning well,” Djokovic said. “I wasn’t making him move too much. I was making way too many unforced errors.
“From 4-4 in the first set, my game fell apart. But if we put things in perspective it has been an amazing year and a great comeback.”