The 36-year-old Swiss beat both men in his opening matches, and with Marin Cilic losing to the duo, Sock and Zverev will face a sudden death clash to join the world No. 2 in the last-four.
Federer will qualify as group winner regardless of the outcome of his result against Cilic and he can enjoy a relaxed afternoon as he follows Henri Kontinen and John Peers’ final match against Nicolas Mahut and Pierre-Hugues Herbert.
Cilic will be playing for pride as he looks to improve on a record that has seen him win just one match in his eight clashes at the O2, but he cannot book his spot in the next round.
The evening session kicks off with a doubles contest between Jean-Julien Rojer and Horia Tecau against Ryan Harrison and Michael Venus, before Sock will take on Zverev in a winner-takes-all tie.
Here’s the full schedule:
ATP Finals Day 5 schedule
12pm: Kontinen/Peers v Herbert/Mahut
NB 2pm: Federer v Cilic
NB 6pm: Rojer/Tecau v Harrison/Venus
NB 8pm: Zverev v Sock