Japan's great hope for tennis gold Naomi Osaka was beaten in the third round of the Tokyo 2020 Olympic tennis singles tournament in a blow to home spirits.

The world number two was swiped aside 6-1 6-4 by Czech Marketa Vondrousova in an early shock on Day Four of the Games.

The tennis event has now lost both of its top draw cards after world number one and Wimbledon champion Ash Barty was beaten in Sunday's first round.

Osaka, who lit the Olympic cauldron on Friday to open the Games, seemed out of sorts on the Tokyo blue courts and never settled into a rhythm as Vondrousova never looked back.

The Japanese player sprayed a costly 18 unforced errors throughout the match -- three times the number made by Vondrousova.

The four-time grand-slam champion did not move as well as she can, perhaps a legacy of the eight-week break she took for mental health reasons prior to this tournament.

Osaka said: "I definitely feel like there was a lot of pressure for this. I think it's maybe because I haven't played in the Olympics before and for the first year (it) was a bit much. I think I'm glad with how I played, with taking that break that I had.

"I've taken long breaks before and I've managed to do well. 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."

Osaka initially appeared to have left the site without talking to the media - in her social media post ahead of the French Open where she announced she would not be talking to the press she likened press conferences after losses to kicking someone when they are down.

However, she returned to answer questions and admitted it was a painful defeat, saying: "I'm disappointed in every loss, but I feel like this one sucks more than the others."

Osaka had won 25 of her last 26 matches on hard courts, winning grand slam titles at the US Open and Australian Open, and, particularly given the early defeat for Wimbledon champion Ashleigh Barty, the stage seemed set for her to claim arguably the biggest title of them all.

But instead it was 22-year-old Vondrousova who reached her first Olympic quarter-final, where she will take on either Paula Badosa of Spain or Argentina's Nadia Podoroska.

She will now turn her attentions to the defence of her title in New York, although Osaka added: "I am a person who wings a lot of things. That is either a really good thing or a really bad thing."

Stefanos Tsitsipas gained revenge for his Wimbledon loss to Frances Tiafoe by beating the American 6-3 6-4.