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.

Federer, 38, will face Serbian world number two Djokovic on Thursday, a repeat of July's marathon Wimbledon final.

The top two players in each group after the round-robin stage progress to the semi-finals on Saturday.

The men's season-ending event, which is taking place in London for the penultimate year, features the top eight players of the year.

Britain's Salisbury picks up first win

Earlier on Tuesday, Britain's Joe Salisbury and American partner Rajeev Ram also kept alive their men's doubles semi-final hopes with victory over Ivan Dodig and Filip Polasek.

The fourth seeds won 3-6 6-3 10-6 after losing Sunday's opener to Marcelo Melo and Lukasz Kubot in London.

Salisbury, playing at the ATP Finals for the first time, said: "We needed to win to stay in it."

He is the only Briton competing at the season-ending event at the O2 Arena.

"I think we will have to win the next one as well, but we're feeling better. The crowd's support was incredible," Salisbury, 27, added.