Japanese superstar Naomi Osaka crashed out of the Tokyo Olympics tennis with a 6-1 6-4 defeat to Czech world number 42 Marketa Vondrousova.

Osaka, who lit the flame at Friday's opening ceremony, had won her opening two matches but bowed out in straight sets, with Vondrousova reaching the quarter-finals.

In the first set, Vondrousova dominated on her first serve and broke three times, before the Japanese hit back with an early break in the second. But the Czech fought back to win, with Osaka finishing with 32 unforced errors to just 10 by her opponent.

"Of course it's one of the biggest wins of my career," Vondrousova said. "Naomi is a great player, she has so many grand slams, so I knew it would be a tough match. I'm very happy with my play. I played amazingly in the first set, and then the second set was really tough. I'm just happy to be through."

World number two Osaka joins top seed and Wimbledon champion Ash Barty in being eliminated early at the Tokyo Games.

In the men's singles second round, fourth-ranked Greek Stefanos Tsitsipas eased past American Francis Tiafoe 6-3 6-4, avenging a shock Wimbledon defeat.

DUFFY DELIGHT

Flora Duffy won Bermuda's first-ever Olympic gold medal with victory in the women's triathlon on Tuesday at Odaiba Marine Park.

The four-time Olympian failed to finish in Beijing, came 45th in London and improved to eighth in Rio de Janeiro.

Duffy took the Tokyo title in a time of one hour, 55 minutes and 36 seconds, finishing more than a minute ahead of Britain's Georgia Taylor-Brown who took silver, with USA's Katie Zaferes claiming bronze.

“It's always been my dream to race at the Olympics and be a professional athlete with the goal of being an Olympic champion," Duffy said.

"That's not the easiest thing to do regardless of where you're from. Bermuda is a small country, but it's really passionate about its sport.

“I'm so grateful that I could achieve a personal dream here of winning an Olympic medal, but this is bigger than me, this is going to inspire the youth of Bermuda and everyone back home that competing on the world stage from a small island is really possible."

The 33-year-old broke clear in a group of seven early in the bike stage, before dominating the 10km run.

Bermuda, which has a population of just over 70,000, had only claimed one medal previously in Olympic history, a bronze in 1976.

MCKEOWN BREAKS OLYMPIC RECORD

Australia secured a second gold medal in the pool as Kaylee McKeown broke the Olympic record in the women's 100m backstroke.

McKeown won in 57.47, finishing ahead of Canada's Kylie Masse by 0.25 seconds, with USA's Regan Smith taking bronze. Masse had led at the turn.

In the men's 100m backstroke, Russian Olympic Committee claimed a one-two finish as Evgeny Rylov edged out countryman Kliment Kolesnikov, who holds the 50m world record. Ryan Murphy, who is the world record holder across 100m, claimed bronze.

Great Britain's Tom Dean won the men's 200m freestyle by 0.04, marginally ahead of compatriot Duncan Scott, with Brazil's Fernando Scheffer coming in third. The result meant two British male swimmers stood on the same Olympic podium for the first time since 1908.

World record holder Lilly King finished third as US teenager Lydia Jacoby triumphed in the women's 50m breaststroke. South Africa's Tatjana Schoenmaker claimed silver.

CHINESE PAIR ON TARGET

China claimed another gold medal in the final of the 10m air pistol mixed team at the Asaka Shooting Range.

China's Ranxin Jiang and Wei Pang won 16-14 over Russian Olympic Committee pair Vitalina Batsarashkina and Artem Chernousov.

Ukraine duo Olena Kostevych and Oleh Omelchuk won the bronze by beating Serbia.

The 10m air rifle mixed team bronze and gold medal matches were taking place later on Tuesday.

AUSSIES SINGING IN MEN'S HOCKEY

World number two side Australia knocked off reigning Olympic gold medallists Argentina 5-2 in the men's hockey group stage.

Australia claimed their third consecutive victory and top Group A, this time being aided by two goals from Blake Govers.

The Kookaburras have only won one Olympic gold despite often being a dominant side in men's hockey and are one of the favourites to triumph in Tokyo.

Japan and New Zealand, who both previously lost to Australia, drew 2-2 in the other Group A game.

In Group B, world number four India won 3-0 over Spain, while fifth-ranked Germany beat Great Britain 5-1.