But Kei Nishikori has kept Japan's tennis medal hopes alive with a tough 7-6 (7-5) 3-6 6-1 second round win over Marcos Giron of the United States in the men's singles.

The 23-year-old Osaka had hoped to play a starring role in the Games on the court as well as off it but lighting the Olympic flame at the opening ceremony will remain the high point.

Osaka had looked good in her first two matches but former French Open finalist Vondrousova was a step up in level under the roof at the Ariake Tennis Park, winning 6-1 6-4.

"I'm disappointed in every loss, but I feel like this one sucks more than the others," Osaka said.

As a four-time grand slam champion, the highest-paid female athlete in the world and the host country's poster girl for the Games, the huge expectations were hard to handle.

"I definitely feel like there was a lot of pressure for this," Osaka said.

"I think it's maybe because I haven't played in the Olympics before and for the first year (it) was a bit much."

The second-ranked Osaka won her opening two matches in straight sets following a two-month mental health break after withdrawing from the French Open, revealing she was dealing with depression and anxiety.

"I've taken long breaks before and I've managed to do well," said Osaka, who initially did not comment after her loss, then came back out and met with a small group of reporters.

"I'm not saying that I did bad right now, but I do know that my expectations were a lot higher.

"I feel like my attitude wasn't that great because I don't really know how to cope with that pressure, so that's the best that I could have done in this situation."

Vondrousova's quarter-final opponent will be Paula Badosa of Spain.

In other women's singles results, fourth seed Elina Svitolina of Ukraine defeated Maria Sakkari in three sets.

Svitolina's quarter-final opponent will be Camila Giorgi of Italy, who defeated Wimbledon finalist Karolina Plisova.

Also reaching the last eight were seventh-seeded Garbine Muguruza of Spain, No.9 Belinda Bencic of Switzerland and No.13 Anastasia Sergeyevna Pavlyuchenkova of ROC.

Meanwhile Nishikori, who played on Centre Court hours after Osaka, said he had not spoken to his teammate since her loss but offered his sympathies.

"Very sad, of course (that) Naomi lost, and surprised. But I knew she had a lot of pressure, this is her first time Olympics and I know it's not easy," Nishikori said.

Nishikori will fly the Japanese flag in the last 16, next taking on Ilya Ivashka of Belarus who defeated Kazakhstan's Mikhail Kukushkin in three sets.

In other men's results, third seed Stefanos Tsitsipas of Greece beat Frances Tiafoe of the United States 6-3 6-4 to also reach the last 16.

He will next take on France's Ugo Humbert who outlasted Serb Miomir Kecmanovic in three sets.

Briton Liam Broady upset Roger Federer's Wimbledon quarter-finals conqueror, Poland's Hubert Hurkacz 7-5 3-6 6-3, booking a third round clash with France's Jeremy Chardy.

Eighth seed Diego Schwartzman of Argentina and No.12 Karen Khachanov of ROC also advanced.

- with Associated Press

Australian Associated Press