Rafael Nadal began his bid for the biggest title missing from his vast collection with an ATP Finals win over Russian debutant Andrey Rublev.
Spanish second seed Nadal, 34, won 6-3 6-4 as the season-ending event began in London for the final time.
Earlier on the opening day, third seed Dominic Thiem avenged his defeat by Greece's Stefanos Tsitsipas in last year's final with a three-set victory.
US Open champion Thiem won 7-6 (7-5) 4-6 6-3 at an empty O2 Arena.
Nadal's straight-set success moved him above the Austrian at the top of Group London 2020 - named in tribute to the event's 50th anniversary - after the opening matches.
Seeing off in-form Rublev - who has won more matches and more titles than anyone else on the ATP Tour this year - was the perfect start for Nadal.
"I am very pleased with the victory, it changes the perspective of the tournament a little bit," said Nadal, who lost in the 2010 and 2013 finals.
"I know I have a super difficult match against Dominic but a win in two sets helps for the confidence."
Only one of the left-hander's 86 career titles has come on an indoor hard court, but he shifted up a gear from his displays in similar conditions at this month's Paris Masters - where he lost in the semi-finals - to frustrate Rublev.
Eighth seed Rublev struggled on serve in the first set - admitting it was a sign of nerves on his Finals debut - and was punished when Nadal earned the decisive break for a 4-2 lead, which he did not relinquish.
Rublev showed his annoyance at his performance by thumping his racquet against the court and more woe came the 23-year-old's way when Nadal broke in the opening game of the second set.
"When he broke me in the second set I started to relax but by then it was too late," Rublev said.
From that point, it seemed almost inevitable the 20-time Grand Slam champion would wrap up a straight-set win and duly did without even facing a break point.
"My serve worked really well, that always helps on this surface and I had a few chances on the return, which I converted," added Nadal, who won 81% of his service points and converted two of five break points.