Novak Djokovic kicked off the 2019 ATP World Tour Finals on Sunday with a straight-set win over Matteo Berrettini.
In the evening match at the O2 Arena in London, Dominic Thiem overcame Roger Federer in a hard-fought two-set win.
Rafael Nadal will be among those in action on Monday.
(2) Novak Djokovic bt. (8) Matteo Berrettini 6-2, 6-1
(5) Dominic Thiem bt. (3) Roger Federer 7-5, 7-5
(4) Daniil Medvedev vs. (6) Stefanos Tsitsipas, 2 p.m. GMT (9 a.m. ET)
(1) Rafael Nadal vs. (7) Alexander Zverev, 8 p.m. GMT (3 p.m. ET)
Djokovic needed just 63 minutes to dispatch Berrettini in the opening match of the tournament, which is reserved for the top eight singles players and doubles teams on tour.
The Serb set the tone by winning the first six points of the contest, and he broke the Italian five times over the course of the two sets.
Berrettini was only able to penetrate his opponent's serve once, though he did come out on top in perhaps the best rally in the match:
Tennis writer Tumaini Carayol felt Djokovic's comprehensive win was an ominous sign for the rest of the field:
Thiem came into his match with Federer having won both of their previous meetings in 2019.
He broke the Swiss in the opening game of their contest on the way to a 2-0 lead, though Federer quickly hit back to level proceedings. Thiem broke again late on in the set after a poor forehand and a long backhand from Federer at 30-30.
Sports journalist Uche Amako praised the Austrian at the end of the first set:
Uche Amako @UcheAmako
Excellent set of tennis from Dominic Thiem. He dominated Roger Federer from the back of the court. The Swiss unable to get enough free points on serve & it cost him. Uphill task now. Federer needs to win this to avoid having to beat Djokovic later in the week #NittoATPFinals
Federer started the second brightly:
The set remained on serve, but Thiem increasingly turned the screw, and the pressure told when he broke for a 6-5 lead.
After saving two break points in the following game, Thiem secured a deserved win when Federer failed to return his serve.