Former world number one Novak Djokovic defeated second seed Stefanos Tsitsipas in straight sets in his opening match of the ATP Finals in Turin.
The 35-year-old Serb defeated Tsitsipas of Greece 6-4 7-6 (7-4).
Djokovic is looking to equal Roger Federer’s record of six wins 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).
Sixth-seeded Rublev kept his cool in a final-set tie-break to defeat the fourth-seeded after wasting seven set points in the opening set.
On Sunday, Rafael Nadal’s latest attempt to finally clinch the first ATP Finals title of his career began with a loss to eighth seeded USA Taylor Fritz.
Top-seeded Nadal will face Canadian Felix Auger Aliassime in his second match of the Greens’ group round robin on Tuesday.
The Turin tournament sees eight of the year’s best men’s players split into two groups of four, with the top two advancing to the semi-finals.
Meanwhile, Britain’s Neal Skupski and Dutch partner Wes Koolhof won their first men’s doubles match 7-6 (7-3) 4-6 10-5 against Australian duo Nick Kyrgios and Thanasi Kokkinakis.
The duo enjoyed incredible success in their first year together, claiming seven titles, including victories at the Masters 1000 in Madrid, Montreal and Paris, and ending 2022 as year-end world number one.
Former world number one Joe Salisbury, seeded second with American Rajeev Ram, will face fellow Briton Lloyd Glasspool and Finnish partner Harri Heliovaara on Tuesday.
