ATP Finals: Nadal edged out as defending champ Zverev powers into semi-finals in London (VIDEO)

Reigning ATP Finals champion Alexander Zverev is on course to repeat his surprise success in London from 2018 after his victory over Daniil Medvedev put him into the semis – and eliminated Rafael Nadal in the process.

Twenty-two-year-old Zverev defeated Medvedev 6-4, 7-6 late on Friday night in England to book his place in Saturday's semi-finals at The O2 Arena, where he will take on 26-year-old Austrian Dominic Thiem in the final four.

The end-of-year tournament in London has proved to be a tough one for the game's biggest stars, with both Nadal and Novak Djokovic crashing out after the round-robin group stages of the competition.

It leaves Swiss star Roger Federer as the highest-ranked remaining player in the tournament, and the world No. 3 will look to extend his incredible record in the ATP Finals as he seeks his SEVENTH tournament victory.

Federer will take on Stefanos Tsitsipas, despite the 21-year-old Greek ace's defeat to Nadal in a thrilling final group game. Nadal, along with countless longtime tennis fans, was hoping to set up a clash with his old sparring partner Federer in the semis, but needed Medvedev to defeat Zverev to secure his passage through to the final four.

But Zverev sealed victory with a strong performance against the Russian to ensure his place in the semis, leaving Nadal to face an early flight home.

"The young guys have been playing much better tennis than they were maybe last year," said Zverev.

"I think the biggest surprise is actually that Novak is out of the tournament, because ... this court, the conditions, is the best fit for his game.

"The other thing is obviously Rafa. Nobody expected him to be out from our group. Our group, for me and Stefanos to qualify, I don't think a lot of people would have picked that."

Zverev's record against Thiem isn't the best, with the German trailing 2-5 in head-to-head meetings between the pair, and the defending champion admitted he is expecting a tough matchup in what will be their first meeting in 2019.

"I hope it will be a back-to-back for me, but I have a very difficult semi-final," he said.

"He's been playing some unbelievable tennis, maybe the best tennis that we have ever seen from him."