Former world number one Novak Djokovic beat second seed Stefanos Tsitsipas in two tight sets in his opening match of the ATP Finals in Turin.
Djokovic is seeking to equal Roger Federer's record of six victories in the season-ending tournament.
Earlier in the same group, Russian Andrey Rublev came from a set down to beat compatriot Daniil Medvedev 6-7 (7-9) 6-3 7-6 (9-7).
Rublev, seeded sixth, held his nerve in a final-set tie-break to overcome the fourth seed after squandering seven set points in the opening set.
On Sunday, Rafael Nadal's latest bid to finally land the first ATP Finals title of his career started with a defeat by American eighth seed Taylor Fritz.