Japan's Naomi Osaka powered through her opening match as women's No 1 Ash Barty stumbled out of the tennis tournament after a lackluster, error-strewn performance.
Osaka, who is aiming to become only the fourth singles player since 1988 to win Olympic gold on home soil, admitted to having the jitters but showed no signs of pressure in dismantling China's Zheng Saisai 6-1, 6-4 on center court.
"I felt really nervous being in Japan and playing here for the first time in maybe two years, and for it to be my first Olympics," Osaka said. "It was definitely nerve-wracking. But I am glad I was able to win. She is a very tough opponent."
Australian Barty, a two-time Grand Slam champion fresh off her victory at Wimbledon, was dumped out of the women's singles after a stunning 6-4, 6-3 loss to 48th-ranked Sara Sorribes Tormo of Spain.
Barty never looked comfortable against the Spaniard, who took advantage of a string of errors off the 25-year-old's racket to set up a meeting with France's Fiona Ferro.
"The key to my game is serving well and I wasn't able to do that. I was a bit erratic," said Barty.
Meanwhile, Britain's Andy Murray, who had been aiming to become the first player to win three Olympic singles gold medals, pulled out of the tournament due to a thigh strain.
He was replaced by Australian Max Purcell, who upset ninth seed Felix Auger-Aliassime of Canada.