Novak Djokovic has made the ideal start in his quest to win a record-equalling sixth ATP Finals title.
The world number two easily beat Italy's Matteo Berrettini in straight sets at the O2 Arena in London.
However, things weren't as rosy for Roger Federer, who was beaten by Dominic Thiem in his tournament opener.
It's the second year in a row that the seven-time Dubai Duty Free Tennis Championships winner has lost his opening match at the ATP Finals.

Pogacar wins third Flanders crown but faces fine for running red light
Thuram and Martinez star as Inter dismantle Roma
Silva to exit Man City at end of season, assistant manager confirms
Penalty heroics help Leeds beat West Ham to reach FA Cup semis
Pogačar wins Tour of Flanders for record-equalling third time
