Simone Biles pulled out of a second event at the Tokyo Olympics on Wednesday, withdrawing from the individual all-around competition to concentrate on her mental health.

Less than 24 hours after she had sent shockwaves around the world by dropping out of the team event, having completed just one vault, the American superstar said she would not take part in Thursday's final, USA Gymnastics said.

Jade Carey will take the 24-year-old's place in the all-around event.

"We wholeheartedly support Simone's decision and applaud her bravery in prioritising her well-being," USA Gymnastics said.

"Her courage shows, yet again, why she is a role model for so many."

The governing body for the sport in the U.S. said Biles would be evaluated to see if she can take part in individual apparatus competitions still to come.

On Tuesday Biles picked up her sixth Olympic medal for being part of the U.S. squad that went on to place second behind Russia.

International Olympic Committee spokesperson Mark Adams had said earlier on Wednesday the IOC has "huge respect and support" for Biles.

Adams said mental health remained a big issue and that it was a matter the organisation had been working on for some time.

IOC President Thomas Bach was seen in deep conversation with Biles on the floor of the Ariake Gymnastics Centre on Tuesday before handing out medals.

Biles was to appear in all six gymnastics event finals and a sweep of the gold medals would have given her 10, making her the most accomplished woman Olympian in any sport.

Netherlands, Britain breeze to wins in women's hockey

The Netherlands secured an easy victory over South Africa in hot, blustery conditions at the Tokyo Olympics on Wednesday, while Britain defeated India and Spain won even as their head coach Adrian Lock remained in COVID-19 quarantine.

The Dutch women's team led by a goal at half-time, striker Frederique Matla having scored after picking up a pass by Lidewij Welten from the left side of the striking circle.

The Dutch side motored through the second half, netting a further four goals in a comfortable 5-0 win. The in-form Dutch women remain undefeated in Tokyo, scoring a total of 14 goals and conceding only one in their three matches so far.

"There are always areas every game where we can improve," said Eva de Goede, playing her fourth Olympics in Tokyo.

"We scored a lot of goals but if you consider the number of opportunities we created, it's actually not that many ... Penalty corners can also be handled better."

With temperatures rising as the game progressed, officials had extended the break time between the third and fourth quarters to four minutes from the usual two to give both sides ample opportunity to cool down.

Spain secured their first triumph of the Tokyo Olympics following a tense match with New Zealand, beating the Black Sticks 2-1.

The Spanish team, whose head coach Adrian Lock was following the game remotely after going into quarantine for COVID-19, led 2-0 at halftime, and managed to retain their lead in the second half as their opponents stepped up the pressure.

Spanish captain Maria Lopez Garcia said the team's first win at these Olympics was important, and the side celebrated by taking a team picture on the pitch after the match.

"Now we have more confidence than before," Lopez Garcia said. "It's a step (up) for the rest of the games."

Elsewhere, Germany won 4-2 against Ireland, while 2016 Rio gold medallists Britain beat India 4-1.

Following an early goal by playmaker Hannah Martin, the Indian squad pulled level with their opponents before falling behind in the 19th minute when Martin bagged her second.

The British team ran out comfortable winners in the end, scoring twice more in the second half.

"Momentum's key," Martin said. "As a squad, we are hoping to take this momentum forward into the Dutch game tomorrow."