Third-seeded Stefanos Tsitsipas of Greece made his Olympics debut with a hard-fought 6-3, 3-6, 6-3 win over Philipp Kohlschreiber of Germany on Sunday in the first round of play in the 2020 Tokyo Games.
Tsitsipas rallied from a break down in the third set to set up a second-round showdown with American Frances Tiafoe, who advanced with a 6-3, 6-2 win over South Korea's Sunwoo Kwon.
Tiafoe ousted Tsitsipas in the first round at Wimbledon in late June.
Germany's Alexander Zverev, the No. 4 seed, had nine aces in a 61-minute, 6-1, 6-3 romp over Yen-Hsun Lu of Chinese Taipei.
Local favorite Kei Nishikori also opened with a victory, topping the fifth seed, Russian Andrey Rublev 6-3, 6-4.
Britain's Andy Murray, the two-time defending Olympic Gold medalist, was forced to withdraw with a right quad injury. He had been scheduled to face Canada's Felix Auger-Aliassime, the ninth seed. Murray's replacement, Australian Max Purcell, upset Auger-Aliassime 6-4, 7-6 (2).
Among others advancing Sunday were Hubert Hurkacz of Poland, Britain's Liam Broady, Russia's Karen Khachanov and Argentina's Diego Schwartzman.
Norway's Casper Ruud topped Hugo Gaston of France 6-3, 6-2 to win his third title of the season in Gstaad.
The No. 3 seed converted four of 16 break points in the one-hour, 36 minute victory.
Gaston, 20, was playing in his first career final.
--Field Level Media