Dominic Thiem beat defending champion Alexander Zverev on Saturday to join Roger Federer's conqueror Stefanos Tsitsipas in the final of the ATP Finals in London.

The Austrian fifth seed saw off the big-serving German 7-5, 6-3 after Greece's Tsitsipas had earlier beaten Federer 6-3, 6-4.

Zverev, who beat Novak Djokovic in last year's final, went toe to toe with Thiem in a relatively uneventful first set but crumbled in the final game, serving a double fault to lose the opener 7-5.

The seventh seed settled back into his serving rhythm at the beginning of the second set but succumbed to another break in the sixth game.

Thiem, looking unflustered, fended off a couple of break points in the following game and served out to take the match, winning with a forehand down the line.

"This is a big, big dream coming true for me, it is one of the biggest and most prestigious tournaments of the whole year and I'm getting the chance to play the final," said Thiem.

Earlier, Tsitsipas, 17 years younger than Federer, ended the 38-year-old's hopes of securing a seventh year-end title.

Federer was unrecognizable from the player who dominated Djokovic in his final round-robin match, struggling on serve and hitting a total of 26 unforced errors compared with just five against the Serbian.

But Tsitsipas belied his years with a performance full of confidence and grit, saving 11 out of 12 break points during the match.

"I'm so proud of myself today, a great performance and once again the people were great," he said.

"I really enjoyed myself on the court and sometimes in matches like these you wonder how you recover from difficulties and break point down.

"It is a mental struggle and I'm proud how many I saved today, I was trying not to give an easy time to Roger. He was playing well."

Tsitsipas conceded a break point in his first game as cries of "Let's go Roger, let's go" rang around London's O2 Arena but he survived the scare and broke Federer in the next game, taking advantage of two missed overheads from the Swiss.