Novak Djokovic made a winning start at the ATP Finals but Roger Federer was beaten by Dominic Thiem in his opener.
Djokovic, who is chasing a record-equalling sixth title and trying to overhaul Rafael Nadal as world number one, was imperious, beating Italian eighth seed Matteo Berrettini 6-2 6-1.
But Federer lost 7-5 7-5 against fifth seed Thiem at London's O2 Arena.

The Swiss, 38, was uncharacteristically wayward while Thiem impressed with his powerful hitting.
Federer and Djokovic meet later in Group Bjorn Borg, their first match since theirmarathon Wimbledon final last July and one Federer may now have to win to reach the semi-finals of the season-ending championships.
Djokovic can overtake Nadal and become year-end number one this week but must reach the final to stand a chance.
Nadal is in Group Andre Agassi but is an injury doubt for the tournament with a stomach muscle issue, although the Spaniard said on Friday that he is confident of playing against Alexander Zverev in his opener on Monday.