Wimbledon champion Ashleigh Barty became the first big casualty of the Olympic tennis tournament.

Two weeks after defeating Karolina Pliskova to win her second grand slam title, the Australian was beaten 6-4 6-3 by Spain's Sara Sorribes Tormo in the first round.

Barty’s disappointment was evident as she walked through the mixed zone with head down, not stopping to talk to reporters.

In quotes released by the Australian Olympic Committee, the 25-year-old said: "I’m disappointed I wasn’t able to get through today, it just wasn’t my day. Credit to Sara, though, she is always a tough competitor.

"I never really felt comfortable out there and wasn’t able to play the match on my terms. The key to my game is serving well and I wasn’t able to do that today. I was a bit erratic and made too many errors."

Barty still has a chance of a medal with Storm Sanders in the women’s doubles, while she is also likely to enter mixed doubles.

It was a huge result for Sorribes Tormo, who has been having an excellent season and beat a world number one for the first time on another day of intense heat and humidity in Tokyo.

Novak Djokovic called for matches to be played later in the day after his first victory but, although the schedule remained the same on Sunday, a concession was made with longer breaks at changes of ends as part of the extreme weather policy.

Naomi Osaka returned to the tennis spotlight on and off the court with victory over China's Zheng Saisai.

Two days after lighting the Olympic flame to cement her status as the face of the Games, Osaka was out in an empty stadium for a 6-1 6-4 victory.

The 23-year-old last played a match eight weeks ago before pulling out of the French Open on mental health grounds after announcing she would not be taking part in press conferences.

Kei Nishikori made home advantage count despite the lack of fans with a 6-3 6-4 victory over fifth seed Andrey Rublev while ninth seed Felix Auger-Aliassime was knocked out in straight sets by 190th-ranked Australian Max Purcell, who stepped in as a replacement when Andy Murray withdrew.

Third seed Stefanos Tsitsipas looked in serious trouble when he went a break down early in the decider against German veteran Philipp Kohlschreiber but the Greek recovered to win 6-3 3-6 6-3.

Pliskova and third seed Aryna Sabalenka would both have been grateful for quick wins – Pliskova beating Alize Cornet 6-1 6-3 and Sabalenka easing past Magda Linette 6-2 6-1.

There were mixed fortunes for Elina Svitolina and Gael Monfils, who flew out to Tokyo to be part of the Ukrainian and French teams, respectively, just days after their wedding.

Fourth seed Svitolina battled to a 6-3 5-7 6-4 victory over Laura Siegemund of Germany but Monfils was beaten 6-4 4-6 7-5 by Ilya Ivashka from Belarus.

Carla Suarez Navarro claimed a joyous first victory of her comeback following successful treatment for cancer, defeating Wimbledon quarter-finalist Ons Jabeur 6-4 6-1.

"I am really happy," said the Spaniard, who will retire later this year. "For me, it was tough all these months. I was so happy on the court and I really enjoyed this week. This is a really special event for me, for us, for everyone. I am really happy for my first win after the comeback.

"It was difficult, but I was practising a lot. I was trying to win at Roland Garros and Wimbledon but it was not possible. But here I am feeling really good, and I played a really good match today, and I am looking forward to the next one."

Fourth seed Alexander Zverev swatted aside Lu Yen-hsun 6-1 6-3, and there were also wins for Hubert Hurkacz, Diego Schwartzman, Petra Kvitova and Garbine Muguruza.