London (CNN) Stefanos Tsitsipas said he would love to have the fan base of Roger Federer one day. That would take some doing.

But on Sunday, the fans were chanting Tsitsipas' name as the Greek all-rounder became the youngest player since John McEnroe 41 years ago to win the ATP Finals on his tournament debut.

The 21-year-old rallied past Dominic Thiem, who was also well supported, 6-7 (6-8) 6-2 7-6 (7-4) in two-and-a-half hours in London to cap a remarkable breakthrough season.

"I remember watching this event on TV and thinking, 'Oh, these guys have done an insane year to be playing here,'" Tsitsipas told reporters. "And now I'm in the position to be champion, so it feels awesome."

Tsitsipas had been a hitting partner for Austria's Thiem at the tournament three years ago and on Sunday he took advantage of a second-set lapse from his friend, and mostly held firm on serve.

In the battle of the one-handed backhand artists, his 34 winners and 16 unforced errors were more solid than Thiem's 34 and 40. In the final two sets, Tsitsipas tallied 20 winners and a mere four unforced. Both men flourished at the net on the fast indoor hard court.

And the winner is... Stefanos Tsitsipas and he can hardly believe it.

Tsitsipas began 2019 as a "Next Gen" prospect but ends it as a real grand slam threat next year.

He has beaten the "Big Three" of Novak Djokovic, Rafael Nadal and -- twice -- Federer, including in Saturday's semifinals when almost all of the 18,000 inside the O2 Arena heavily backed the 38-year-old.

Navigating through a two-week grand slam has been too much so far for the younger guard to overcome but unlike last year's 21-year-old winner in London, Alexander Zverev, Tsitsipas already has reached a grand slam semifinal, in Melbourne.

And let's not forget 23-year-old Daniil Medvedev, the big-serving, counter-punching US Open finalist.

Thiem and Zverev said this week they expected someone to break through next year.

Tsitsipas certainly didn't discount himself.

"I feel like my game is getting better over time, and I believe I'm really close on being crowned a grand slam champion," said Tsitsipas. "I know these are strong words that I say, but I do feel like I belong to be there."

But later he acknowledged how difficult getting the better of the "Big Three" would be.

"The thing that we have the Big Three dominating the grand slams the last couple of years makes it really difficult for us, because someone needs to get the job done in order to defeat them early rounds, because once they get deep in a tournament, they tend, as we saw over the years, to play, to get better and play better," he said.

"So for me, that's a really difficult task to do for players, to be able to beat them in these grand slams, because it's the best-of-five format, and it just gives them more chances to stay in the match.

"For us, for the young guys, it's all about time. We will have to beat them or wait for them."

Tsitsipas bookended 2019 with victories over Federer -- the first coming at January's Australian Open when he urged fans to subscribe to his You Tube channel. They obliged, so his numbers indeed grew there.

Now, however, Tsitsipas appears to have cooled on social media.

Stefanos Tsitsipas sinks to the ground after match point at the ATP Finals.

His game had no problem heating up in London, where he was watched by dad Apostolos -- who saved him from drowning in 2015 -- and another of his coaches, Patrick Mouratoglou. The latter is likely best known for also working with 23-time grand slam winner Serena Williams.

Break points saved

Tsitsipas rebounded from a nearly three-hour defeat to Nadal in his final group match Friday to upend Federer, saving 11 of 12 break points.

And he didn't show any signs of weakening in the first set against Thiem, who had topped both Federer and Djokovic in group play.

Tsitsipas saved a combined 18 of 21 break points against Federer and Nadal and fended off both in the first set in the seventh game.

The odds were with Tsitsipas in the tiebreaks given his 33-12 record in breakers this year but he was never allowed to settle. Thiem never trailed and only once were they level, at 5-5. Tsitsipas saved a first set point at 5-6 thanks in part to a clutch volley but mis-hit a backhand at 6-6.

Thiem ended the set by delivering a huge serve.

Stefanos Tsitsipas and Dominic Thiem (front) exchange a hug after their ATP Finals thriller.

With the opener lasting more than an hour, it felt like a huge blow to Tsitsipas.

But instead of capitalizing on that momentum, Thiem's errors allowed Tsitsipas to break to start the second set.

Major lapse

That minor hiccup turned into a major lapse for the 26-year-old Thiem, who soon trailed 4-0.

Tsitsipas didn't relinquish the advantage, comfortably forcing a decider.

Thiem bravely saved two break points to start the third courtesy of big forehands down both lines but cracked at 1-1 by sending a backhand into the net.

Victory was in sight for Tsitsipas but he blinked at 3-2, erring at 15-all with Thiem out of position. On the second break point in that game, he sent a backhand wide for 3-3. It was a rare blemish.

And the spectators delivered one of their biggest roars of the evening when Thiem lashed a flat one-handed backhand down the line in the ensuing game as part of a love hold.

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The momentum was back with Thiem, so Tsitsipas deserves ample credit for holding to love when serving to stay in the match at 4-5. He is, as Federer put it, "tough as nails."

On the second occasion at 5-6, however, Thiem greatly aided Tsitsipas by erring on a forehand at 0-15.

As the crowd took turns backing one player or the other, the match hung in the balance.

Nervy ending

The final tiebreak was mostly about Thiem, one way or the other. He trailed 4-1, got to 4-4, but then erred on a forehand long with the court open.

"I missed some very close balls in the tiebreak, which against Novak or which in other matches in the last weeks went in," said Thiem, who dealt with illness earlier in the event. "That was the thing in the third-set tiebreak.

"I cannot do anything now about it. It was a great match from both of us, and he really deserves that win."

Tsitsipas sealed the affair when Thiem sent a return wide before sinking to the ground. The pair then exchanged a hug, a month after Thiem won their battle in the Beijing final.

Despite the reverse, Thiem shouldn't be discounted from breaking through at a major next year, either.

He cut ties with Boris Becker's old coach Gunter Bresnik and teamed with former Olympic gold medalist Nicolas Massu, the result being a transformation from outstanding clay-court player -- Thiem has appeared in two French Open finals -- to outstanding all-round player.

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Of his five titles in 2019, three came on hard courts, including the biggest regular season tournament outside the majors at Indian Wells.

No wonder Federer said Thiem -- in the generation younger than the "Big Three" but older than the "Next Gen" -- was in his prime.

But it wasn't enough to overcome Tsitsipas and now the tennis world awaits his 2020.