Rafael Nadal launched his bid for a first ATP Finals trophy in style on Sunday, taming in-form Andrey Rublev in straight sets in London after Dominic Thiem beat Stefanos Tsitsipas.
The Spanish world No 2, chasing the biggest prize missing from his glittering resume, barely broke sweat as he outclassed the Russian debutant 6-3, 6-4 at the empty O2 Arena.
Earlier, US Open champion Thiem avenged his defeat by Tsitsipas in the championship match last year, beating the Greek sixth seed 7-6 (7/5), 4-6, 6-3.
Nadal, who last month equalled Roger Federer's record of 20 Grand Slams by winning the French Open, has won 86 titles but just one of those has come on an indoor hard court.
His record at the eight-man elite event pales in comparison with his main rivals. Federer, who is absent with injury, has won it a record six times and Novak Djokovic has triumphed five times.
Nadal, 34, and Rublev both held serve comfortably in the early stages of their Group London 2020 round-robin match.
But the Spaniard, who was comprehensively outserving his 23-year-old opponent, made his move in the sixth game, breaking the Russian, who hurled his racket to the ground in frustration, and wrapping up the set with the minimum of fuss.
Rublev, who has won a tour-leading five titles during the coronavirus-interrupted 2020 season, was broken again in the first game of the second set, leaving him with a mountain to climb.
The seventh seed upped his first-serve percentage but it was too little too late.
Nadal did not face a break point in the entire 77-minute match.
"Always the first match is very tricky," said Nadal. "Every match is difficult when you're playing against the best of the best so it's a positive start for me, very happy."
"I think my serve worked very well tonight so that's a bit of help always on these surfaces," added the two-time finalist.
"I had my few chances on the return and I converted."
The French Open champion said winning in straight sets was a big confidence boost ahead of his "super-difficult match" against Thiem on Tuesday.
The Spaniard has qualified for the ATP Finals for a record 16 consecutive years but has been forced to withdraw on six occasions.
A combination of injuries and his preference for other surfaces has counted against him but his greater freshness as a result of a shorter season may boost his chances this year.
In the opening match on Sunday, Austria's Thiem missed a clutch of break points against Tsitsipas but battled back from 4-1 down in the first set tie-break to take the lead.
The defending champion broke early in the second set to level the match and appeared to have the momentum but Thiem rallied impressively, earning a break with a delicately dinked forehand in the second game of the decider, which proved decisive.
"I think the level was higher last year," said Thiem.
"We were both in great shape last year in the final, we wanted that title 100 percent. The atmosphere was insane last year in the final and today was a little bit different."
Earlier, Novak Djokovic received the ATP Tour No. 1 trophy after finishing the year at the top of the rankings for a record-equalling sixth time.
The top seed, who is competing in Group Tokyo 1970 alongside Daniil Medvedev, Alexander Zverev and Diego Schwartzman, plays his opening match on Monday.
The O2 Arena is hosting the ATP Finals for the 12th and final season before the year-end championships moves to Turin next year.
Results from the first day of the ATP Finals at London's O2 Arena on Sunday (x denotes seeding):
Group London 2020
Dominic Thiem (AUT x3) bt Stefanos Tsitsipas (GRE x6) 7-6 (7/5), 4-6, 6-3
Rafael Nadal (ESP x2) bt Andrey Rublev (RUS x7) 6-3, 6-4
Group Mike Bryan
Wesley Koolhof/Nikola Mektic (NED/CRO x5) bt Kevin Krawietz/Andreas Mies (GER/GER x3) 6-7 (3/7), 7-6 (7/4), 10-7
Rajeev Ram/Joe Salisbury (USA/GBR x2) bt Lukasz Kubot/Marcelo Melo (POL/BRA x8) 7-5, 3-6, 10-5