Roger Federer recovered from his opening round-robin defeat to beat Italy's Matteo Berrettini in straight sets at the ATP Finals.
The Swiss 20-time Grand Slam champion, who lost to Dominic Thiem on Sunday, won 7-6 (7-2) 6-3 to keep alive his hopes of a semi-final place.
Federer, seeded third at the O2 Arena, has failed to qualify for the last four on just one occasion in London.
Novak Djokovic takes on Thiem in the other singles match later on Tuesday.
Thiem is the only player who can qualify for the semi-finals at this stage, needing a victory against Djokovic.