Dominic Thiem serves in his semifinal singles match against Alexander Zverev at the ATP World Tour Finals on Saturday in London. Photo: VCG

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, making his debut at the tournament, had earlier beaten Federer 6-3, 6-4.

Thiem is playing in his fourth ATP Finals but during his first three trips to the season ­finale, he won only three matches and never advanced out of the group stage at the O2 ­Arena.

Zverev landed 75 percent of his first serves but Thiem was tougher in the big moments, breaking in the 12th game of the opening set and in the sixth game of the second set.

The Austrian also saved all four break points on his racquet.

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

The seventh seed, 22, regrouped and settled back into his serving rhythm at the beginning of the second set but was broken again to trail 4-2.

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.

The 26-year-old, who reached the final of this year's French Open, has been considered a clay-court specialist but joked after winning indoors in Vienna last month and now reaching the final in London he was becoming a "big, big fan of faster hard surfaces."

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

Federer was unrecognisable 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."

"I'm frustrated I couldn't play better, and when I did and fought my way back, I threw it away again," said the Swiss. "It was also parts of him. He did come up with the goods when he had to and he was better than me today."