Naomi Osaka of Japan serves to China's Zheng Saisai during the women's tennis competition at the Tokyo Olympics on Sunday in Tokyo, Japan. Photo: VCG

Japan's Naomi Osaka won in straight sets on her highly anticipated return to court at the Tokyo Olympics on Sunday, while world No.1 Ashleigh Barty crashed out in the first round.

Osaka, the star of the opening ceremony, defeated China's Zheng Saisai 6-1, 6-4 in her first match since taking an eight-week break for mental health reasons.

"I feel like more than anything I'm just focused on playing tennis. Playing the Olympics has been a dream of mine since I was a kid," said Osaka, chosen as the first tennis player to light the Olympic cauldron on Friday.

"I feel like the break that I took was very needed, but I feel definitely a little bit refreshed and happy again."

The Australian Open champion had not played since May, when she walked out of Roland Garros saying that media commitments were harming her mental health.

Osaka showed few signs of rust as she reeled off the opening five games on the Ariake Coliseum center court, which likely would have been packed to capacity if not for a spectator ban because of coronavirus restrictions.

The second seed broke the 52nd-ranked Zheng for a 2-1 lead in the second set and closed out an 87-minute victory on her third match point.

The 23-year-old Osaka is attempting to become Japan's first Olympic tennis champion. She will play Switzerland's Viktorija Golubic in the last 32. 

Osaka is now the clear favorite for gold after Barty, the reigning Wimbledon champion, dropped serve twice in each set and made 55 unforced errors in a 6-4, 6-3 loss to Spain's 48th-ranked Sara Sorribes Tormo.

"It's an amazing feeling, I still can't believe it," said Sorribes Tormo, who will go on to face France's Fiona Ferro in the second round.

"It's something incredible for me. It's something that I've been dreaming of all my life, being here and even more so beating the world No.1. I'm super, super happy."

Barty fell a double break behind at 1-4 in the opening set and failed to convert two points to level in the eighth game as Sorribes Tormo held firm.

The pair exchanged breaks to begin the second set, but Barty wilted on another hot day in Tokyo and Sorribes Tormo won the final four games to advance.

Defending champion Andy ­Murray withdrew from the men's singles tournament due to a quad muscle strain just hours before he was scheduled to play Felix Auger-Aliassime.

Murray, 34, will stay in Japan to play doubles with Joe Salisbury. The pair won their opening match of the competition on Saturday.

"I am really disappointed at having to withdraw but the medical staff have advised me against playing in both events, so I have made the difficult decision to withdraw from the singles and focus on playing doubles with Joe," Murray said in a Team GB statement.

Three-time Grand Slam champion Murray is the only player to win two Olympics singles titles (2012 and 2016). He was replaced in the draw by Australia's Max Purcell.

Karolina Pliskova, the fifth seed and recent Wimbledon runner-up, demolished France's Alize Cornet 6-1, 6-3, while Aryna Sabalenka, the third seed from Belarus, made quick work of Pole Magda Linette 6-2, 6-1.