Biles won four gold medals in a dazzling 2016 Rio Olympics campaign, but she withdrew from multiple events at the Tokyo Games as she battled the disorientating condition that gymnasts call "twisties".

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

After taking two years out, Biles made a triumphant comeback last year, earning four world titles to take her tally to an astonishing 23.

She 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.

The buzz was palpable at Bercy Arena, where celebrity fans included Hollywood A-listers Tom Cruise, Jessica Chastain and Greta Gerwig, pop star Ariana Grande, Vogue editor-in-chief Anna Wintour and Olympic snowboard star Shaun White and his wife, Canadian actress Nina Dobrev.

Chants of "U-S-A" began to ring out even before gymnasts made their entrance for the second sessin of qualifying, and the roar when Biles was introduced was deafening.

When she saw that her image was on the giant scoreboard screen Biles gave a small wave and kiss to the crowd.

She didn't disappoint once competition got underway, crushing every skill on the narrow beam and exiting to another ovation.