American tennis star Jack Sock has joined legend Roger Federer as semifinalists from the Boris Becker Group at the ongoing ATP Finals in London after his thrilling three-set win over German third seed Alexander Zverev.

Eighth seed Sock saw off Zverev 6-4 1-6 6-4 to finish in second place behind Federer thereby setting up a semi-final meeting with Grigor Dimitrov.

Speaking after his win, Jack Sock commended the fighting spirit he showed against Zverev. He said: “That was a rollercoaster match.”

“He played well in the second, I didn’t have the hottest start in the third, but I kept in there and kept fighting.” Sock added.

Group winner Federer had earlier beaten Marin Cilic 6-7 (5-7) 6-4 6-1 to head into the semi-finals with a 100% record. He had already won the group with victories over Zverev and Sock.

“Knowing I was qualified on Tuesday evening – it’s kind of weird to be already through to the semis, but of course it gives me a great idea how I want to prepare for Saturday.” the 19- time Grand Slam winner said after the match.

“One more last weekend, one more push before the vacation and then preparation for the next season.” he concluded.

The semi-final line-up for Saturday will be completed by the winner of the Pete Sampras Group match between David Goffin and Dominic Thiem on Friday. In the other group match, the impressive Grigor Dimitrov of Bulgaria will face Pablo Busta as he aims to maintain his 100% record.