(CNN) - This has unquestionably been Novak Djokovic's year in men's tennis. But that didn't matter to Alexander Zverev on Sunday as he stunned Djokovic 6-4 6-3 in London to become the youngest winner of the year-end championships since the Serb himself in 2008.
Djokovic had looked destined to become the first man since Zverev's coach, Ivan Lendl, in 1986 to win the World Tour Finals or its predecessors without dropping a set, such has been his glittering form at the O2 Arena and in the second half of 2018 in general.
The newly returned world No. 1 hadn't even dropped serve all week.
He had won 35 of his past 37 matches, including titles at Wimbledon and the US Open to lift his tally of majors to 14 in a remarkable comeback campaign.
Indeed all signs pointed to Djokovic equaling Roger Federer's haul of six year-end crowns -- Federer lost to Zverev in a dramatic semifinal Saturday.
Yet Zverev -- the 21-year-old tipped to be a multiple grand slam winner in the future despite a rocky start to his grand slam career -- was the last man standing to also become Germany's first champion at the event since one of his mentors, Boris Becker, in 1995.
No one before had ever beaten Djokovic and Federer at the same edition of the year-end championships.
"Obviously it's quite astonishing, winning this title, beating two such players back-to-back, Roger and Novak, in the semifinals and finals," Zverev told reporters. "Means so much. I'm incredibly happy and incredibly proud of this moment right now."
While Zverev flourished, Djokovic dipped.
"I was making way too many unforced errors," Djokovic told reporters. "From 4-4 in the first set, my game really fell apart. But credit to him because he played solid.
"Health wise I haven't been really perfect in the last three, four weeks," added Djokovic. "That took a lot out of me.
"Of course finishing the year as No. 1, that was the goal coming into the indoor season. I managed to achieve that. Overall it was a phenomenal season that I have to be definitely very proud of."
The 6-foot-6 Zverev served huge Sunday -- delivering 10 aces -- covered the court terrifically and cleverly altered the shape on his ground strokes, pinning a baffled Djokovic back by hitting with plenty of spin.
When the world No. 4 fell to Djokovic in the group stage 6-4 6-1, the ninth game proved massive.
Djokovic saved two break points, broke in the next game, then cruised.
The ninth game was pivotal again in the title match and on this occasion it went Zverev's way. He broke courtesy of a forehand error and clubbed three aces to begin the ensuing game.
A rattled Djokovic was broken again to start the second but when he broke right back, it felt like a comeback could be on the cards. This was, after all, Djokovic.
It was not to be, however, with Zverev breaking for 2-1 to regain the advantage and never looking back.
He struck a backhand passing shot winner on a second match point, then dropped to the court in joy. Djokovic was soon there to greet him.
Zverev, whose older brother Mischa is a tennis pro, later hoisted the trophy and picked up a check of $2.5 million.
The decider once again showed how difficult it is to beat the same player twice at the year-end championships. It has now only happened eight of 18 times.
"I knew that he's going to change something, he's going to play better and he did," said Djokovic. "I just played under par comparing to all the matches I had so far this week."
There were no boos for Zverev, a day after a section of the roughly 17,500 fans at the O2 Arena got on his case after he defeated Federer 7-5 7-6 (5).
Simply ousting the ever popular Swiss wasn't the reason for the jeers. Rather Zverev stopped play trailing 4-3 in the second-set tiebreak when a ball boy moved to retrieve a ball rolling on court.
Federer held the advantage in the rally but the point was replayed. Zverev duly struck an ace.
Even though he did nothing wrong -- the rules dictate the point should have been replayed -- a flustered Zverev felt the crowd's wrath while being interviewed on court.
"Yesterday I felt like I've done everything right, to be honest, by the rules and how it should have been. But the crowd reaction wasn't too good to me," said Zverev.
"I was a little bit sad because as tennis players, we take it very personal, this kind of stuff. It's a one-on-one sport, so we take things more personal than other sports.
"Today, the mindset, I just wanted to enjoy being out there, I just wanted to enjoy competing and playing against the best player in the world."
He looked like he did enjoy it.
And as for the fans, they were fully supportive Sunday in what was an unexpected conclusion to the season.
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