19-time Grand Slam Champion Roger Federer saw off a spirited challenge from 20-year-old Alexander Zverev of Germany to reach the semi-final of the season-ending ATP World Tour Finals here on 15 November.
After comfortably winning the first set, the Swiss tennis maestro went down in the second before he held young pretender Zverev at bay to clinch a 7-6 (8-6), 5-7, 6-1 win in a thrilling clash that lasted two hours and 11 minutes.
Reflecting on his match against the German, Federer said, "I think I was able to stay the course and use my slice quite effectively, then try with variation to go into his forehand."
The world number two is placed in Boris Becker group, alongside Zverev, Croatian player Marin Cilic and Jack Sock of United States. And Federer insisted though it has been a tough group, but today's match was the most difficult one.
"I'm extremely happy... It's been a tough group so to be there in two matches is great. But today was difficult. Still early days in the tournament. It was nice to be able to show maybe that quality of mine, that I can dig out these matches, these points time and time again, stay mentally tough. In the third I started to play better. It was a tough match from the beginning till the end," the ATP quoted Federer, as saying.
With the win, world number two Federer became the first man to book his place in the last-four at the O2 Arena, while he also stays on course for his seventh end-of-season crown.
Zverev, on the other hand, can still cement his place in the semi-finals on his debut appearance at the season-ending tournament.
He will now lock horns with Jack Sock of America in his final round-robin match on 16 November, with the winner of the clash to join Federer in the last four.