Gymnastics icon Simone Biles dazzled in her long-awaited return to the Olympic stage on Sunday as Paris braced for a blockbuster swimming duel on an action-packed day two at the Games.

A galaxy of A-listers, from Hollywood star Tom Cruise to Vogue editor-in-chief Anna Wintour, crowded into a packed Bercy Arena to watch as Biles began her quest for Olympic redemption.

The 27-year-old four-time Olympic champion opened her qualifying with a flawless performance on the beam, drawing loud applause from a star-studded crowd that also included actress Jessica Chastain, "Barbie" director Greta Gerwig and pop singer Ariana Grande.

Biles, a 23-time world champion widely regarded as the greatest gymnast in history, entered the arena to a standing ovation and a deafening roar greeted her introduction.

The American star, who also impressed with her floor routine, acknowledged the support with a wave and she blew a kiss to the crowd after her image was flashed up on a giant scoreboard.

Biles is competing in Paris three years after her campaign at the Tokyo Olympics unravelled due to a disorientating condition gymnasts call "twisties".

She still went home with a silver and bronze and was lauded for speaking openly about her mental health struggles.

Biles could become the oldest all-around women's all-around Olympic champion in 72 years and just the third woman to capture more than one Olympic all-around title.

- Swimming duel -

Biles' return was one of the highlights of a day two schedule which will see 13 gold medals up for grabs in sports including shooting, mountain biking, skateboarding, archery, canoeing, fencing and swimming.

All eyes will be on the battle for gold in the swimming pool at La Defense Arena, where Britain's Adam Peaty faces a duel with China's world champion Qin Haiyang in the 100m breaststroke.

World record-holder Peaty is trying to match swimming legend Michael Phelps and win a third straight gold medal in the same event in a clash with Qin that is laced with intrigue.

Qin, the 200m world record-holder, was reportedly among the 23 Chinese swimmers who tested positive for a banned drug in late 2020 to early 2021.

Chinese swimming chiefs have denied wrongdoing, saying the swimmers tested positive after inadvertently consuming tainted food during domestic competition.

Peaty's tussle with Qin is one of three swimming golds to be decided on Sunday.

France's great swimming hope Leon Marchand will chase glory in the men's 400m individual medley, while the women's 100m butterfly title will also be on offer.

In other action on Sunday, the United States' star-studded basketball squad spearheaded by LeBron James and Stephen Curry will launch their quest for a fifth straight gold medal against Serbia, who are led by three-time NBA MVP Nikola Jokic.

- Nadal set for return -

Rafael Nadal was meanwhile poised to play in the tennis singles, according to Spanish team captain David Ferrer, despite earlier fears about his fitness.

Nadal, who partnered Carlos Alcaraz in doubles on Saturday, is due to face Hungary's Marton Fucsovics on Sunday, with Novak Djokovic lying in wait for the winner.

After the rain-sodden start to the Games on Saturday, Paris awoke on Sunday to blue skies and sunshine, with a heatwave forecast early next week.

However the improved weather failed to prevent the first triathlon training session in the River Seine from being cancelled on Sunday.

Following a meeting "on the water quality" and tests, "a joint decision was taken to cancel the swimming part of the triathlon orientation," a Paris 2024 and World Triathlon statement said.

Organisers blamed rain in recent days and said they were "confident" the water quality would improve enough in time for the two days of the triathlon competition on July 30-31.

At the beach volleyball fans booed Dutch player Steven van der Velde, a convicted rapist who was controversially selected for the Olympics despite his criminal past.

Van der Velde was convicted in 2016 of raping an underage girl and sentenced to four years in prison.

The Netherlands' decision to pick the 29-year-old sparked outrage in the lead-up to the Games and he is not staying at the athletes' village.

Van der Velde's teammate Matthew Immers spoke out in support of his colleague after the Dutch lost 15-13 to Italy.

"What is in the past is in the past, and he had his punishment," Immers said.