Novak Djokovic made a winning start at the ATP Finals as he chases a record-equalling sixth title and overhaul Rafael Nadal as world number one.
The Serb was imperious, beating Italian eight seed Matteo Berrettini 6-2 6-1.
World number two Djokovic can overtake Nadal to become year-end number one this week in London but must reach the final to stand a chance.
Roger Federer plays Dominic Thiem in the same group later on Sunday as the season-ending championships.
The event at the O2 Arena features the top eight players of the year who are split into two groups.