Superstar gymnast Simone Biles pulled out of a second event to protect her mental health on Wednesday, putting the spotlight on athletes' wellbeing at a Tokyo Olympics held under strict rules to prevent the spread of coronavirus.

Biles, who caused shockwaves with her withdrawal during the team event on Tuesday, also ditched the all-around, raising doubts about her further participation in Tokyo.

Australia's Ariarne Titmus (C), nicknamed "Terminator", toppled 2016 champion USA's Kathleen Ledecky (R) in a thrilling 400m freestyle race
Australia's Ariarne Titmus (C), nicknamed "Terminator", toppled 2016 champion USA's Kathleen Ledecky (R) in a thrilling 400m freestyle race AFP / Attila KISBENEDEK

The 24-year-old American's struggles follow those of Japanese tennis star Naomi Osaka, another face of the Games who lost in the third round on her return from a mental health break.

They overshadowed another busy day at the delayed 2020 Games, where US swimming great Katie Ledecky bounced back from her second defeat by Australia's Ariarne Titmus to win her first gold in Tokyo.

Biles, unbeaten in all-around competition since 2013 and widely touted as the 'G.O.A.T' (Greatest Of All Time), arrived seeking five Olympic titles to equal Soviet gymnast Larisa Latynina's career record of nine.

Hungary's defending champion Katinka Hosszu is seeking to make up for a miserable start to the Olympics
Hungary's defending champion Katinka Hosszu is seeking to make up for a miserable start to the Olympics AFP / Attila KISBENEDEK

But this week she complained she had "the weight of the world on my shoulders" and withdrew after a single, shaky vault in the team competition. Attention will now focus on whether she will compete in her four remaining individual events.

Biles is not alone in suffering mental problems, and several athletes have complained of difficulties during coronavirus lockdowns. Australian basketball star Liz Cambage was one who skipped Tokyo fearing its "terrifying" living conditions.

On Wednesday, Dutch athletes in quarantine in Tokyo after testing positive revealed they staged a sit-in strike over the harsh conditions, eventually earning the concession of being able to stand at an open window for 15 minutes a day.

USA's Simone Biles arrived in Japan as one of the headline acts of the pandemic-postponed 2020 Games, shouldering an immense weight of expectation
USA's Simone Biles arrived in Japan as one of the headline acts of the pandemic-postponed 2020 Games, shouldering an immense weight of expectation AFP / Lionel BONAVENTURE

"Not having any outside air is so inhuman, and it is mentally super-draining," said street skateboarder Candy Jacobs, while taekwondo fighter Reshmie Oogink called it "Olympic jail".

International Olympic Committee spokesman Mark Adams said support measures included psychologists in the Athletes' Village and phone helplines.

Support for USA's Simone Biles (C) poured in from around the world
Support for USA's Simone Biles (C) poured in from around the world AFP / Lionel BONAVENTURE

"That became more urgent obviously with the Covid pandemic, so we've been working on that quite a lot," he said.

Fresh from leading the Russian gymnasts to a narrow men's team win over Japan, world champion Nikita Nagornyy will again face home hope Daiki Hashimoto in the men's all-around competition
Fresh from leading the Russian gymnasts to a narrow men's team win over Japan, world champion Nikita Nagornyy will again face home hope Daiki Hashimoto in the men's all-around competition AFP / Lionel BONAVENTURE

Elsewhere Ledecky, who won four gold medals at the Rio 2016 Games, lost her 200m freestyle crown to Titmus, two days after also ceding her 400m title to the Australian.

But Ledecky regrouped to annihilate the field in the first women's 1500m held at an Olympics, finishing more than four seconds clear of US teammate Erica Sullivan.

Ledecky, 24, is just the fourth female swimmer to claim six Olympic gold medals. However Titmus, nicknamed "Terminator", is emerging as one of the stars at the Tokyo Aquatics Centre.

Japan's Yui Ohashi completed a medley double, winning the 200m event to go with the 400m gold she secured on Sunday, while world record-holder Kristof Milak powered to the men's 200m butterfly gold.

Britain's freestyle swimmers brought the morning session to a thrilling climax, narrowly missing out on a world record in the 4x200m freestyle as they beat the Russian team by more than three seconds.

In baking conditions, Spanish tennis player Paula Badosa retired and left the court in a wheelchair after suffering heatstroke, and Russia's Daniil Medvedev had breathing problems during his third-round win over Fabio Fognini.

In response to the incidents, organisers said from Thursday matches would not start until 3:00pm to avoid the worst of the Tokyo heat.

Elsewhere, Dutch rider Annemiek van Vleuten won the women's cycling time trial, and Fiji retained their men's rugby sevens title with a typically free-flowing 27-12 win over New Zealand.

In men's basketball, the United States bounced back to hammer Iran 120-66 after they fell to a strong French team for their first Olympic defeat in 17 years.

Shi Zhiyong won the men's 73kg weightlifting with a world-record total of 364kg, giving China their fourth gold of the competition.

Japanese gymnast Daiki Hashimoto won the coveted men's all-around title at the age of 19, edging China's Xiao Ruoteng with a superb final horizontal bar routine.

But in a disappointment for the hosts, Japanese top seed Kento Momota fell at the first hurdle in the men's singles badminton, losing in just 52 minutes to unseeded South Korean Heo Kwang-hee.

The United States made history as the first 3x3 basketball Olympic champions when their women beat the Russian Olympic Committee 18-15. Latvia took the men's title.

Reigning champions Brazil cruised into the men's football quarter-finals as Richarlison scored twice in a 3-1 win against Saudi Arabia but France crashed out after they were beaten 4-0 by Japan.