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 their marathon 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.
This is the second year in a row that Federer has lost his opening match at the ATP Finals, having been defeated by Kei Nishikori 12 months ago.
The 20-time Grand Slam champion made a slow start, losing his serve in the opening game of the match, and despite levelling the scores three games later, a string of errors off the Swiss’ racquet at 5-5 saw Thiem take the decisive break of the first set.
Federer struggled to create momentum because of his errors but threatened to take the upper hand early in the second set, only for Thiem to save break point with a brilliant forehand on the run.
The Austrian was on the front foot for much of the match and pushed his opponent in most of his second-set service games before eventually clinching another break for a 6-5 lead thanks to a crunching forehand, a fine return and Federer’s missed volley.
Federer had two opportunities to break back when his opponent served for the match but was unable to convert – and Thiem sealed the win on his second match point.
He has now beaten Federer in each of their past three meetings and leads their overall head-to-head 5-2.
