Continuing to defy Father Time, Roger Federer has seen off generation next to surge into a record 14th semi-final in 15 appearances at the ATP Finals.

The ageless Swiss resisted a sprited challenge from German prodigy Alexander Zverev to close in on an unprecedented seventh season-ending championship with a 7-6 (8-6) 5-7 6-1 victory on Tuesday night.

With Federer safely through to the last four, Zeverev and American Jack Sock will square off on Thursday for the other semi-final berth from their four-man pool in the round-robin format.

Sock earlier on Monday kept his hopes alive after coming from 4-2 down in the final-set tiebreak to overhaul Wimbledon finalist Marin Cilic 5-7 6-2 7-6 (7-4) and claim his first-ever win at the elite eight-man event.

The American — the first from his country to reach the event since Mardy Fish in 2011 — needed 2hrs 28mins to deny the big-hitting Croat in a momentum-shifting match.

“It was a tough one for sure,” Sock said after defeating Cilic for the third time in as many attempts.

“It was an interesting day. We had a 4am fire alarm in our hotel also.

“I’m excited to excited to keep winning and keep my hopes alive this week.”