Roger Federer faces an uphill task to qualify for the last four of the ATP Finals after a 7-5, 7-5 defeat to Dominic Thiem in his tournament opener on Sunday.
The six-time champion was in action in the evening match after Novak Djokovic had swatted aside debutant Matteo Berrettini in the earlier Group Bjorn Borg contest in London.
Federer and Djokovic, who has won the end-of-season event five times, will meet later in the week in a repeat of this year’s Wimbledon final, with two players advancing from the four-man group.
The Swiss 20-time Grand Slam winner recovered quickly from being broken in the first game of the match but the Austrian fifth seed repeated the dose in the 11th game and took the opening set 7-5.
Federer squandered an early chance to break in the second set and the games went with serve until the 11th game again, when Thiem broke to love after the third seed dumped a forehand into the net.
