US gymnastics star Simone Biles slipped off the balance beam and finished out of the medals on the unforgiving apparatus on Monday, ending her bid for a record five golds at the Paris Olympics.

Biles' hopes of a fourth gold medal of these Games -- and an eighth career gold -- now rest on the floor exercise that wraps up gymnastics competition at Bercy Arena.

Biles was one of several gymnasts to fall off the beam, the first hiccup in her triumphant Olympic return three years after she withdrew from most of her events in Tokyo.

The pandemic-delayed Tokyo Olympics saw her suffer famously from the disorientating mental block gymnasts call the "twisties".

In Paris, she has already led the United States to team gold, regained the all-around crown she first won as part of a four-gold haul in Rio in 2016, and soared to the vault title with a stellar rendition of her signature Yurchenko double pike.

But she came fifth in the balance beam, with Alice D'Amato becoming the first Italian woman gymnast to claim Olympic gold with a score of 14.366.

Teammate Manila Esposito joined her on the podium in third, with China's Zhou Yuqin taking silver.

The treacherous nature of the 10cm-wide beam was clear when the first three starters made big mistakes.

China's world silver medallist Zhou lost her balance and had to touch the beam to avoid coming off.

American medal contender Sunisa Lee and Brazilian Julia Soares both fell, Lee taking a hard fall onto the wooden beam when her foot slipped at the end of what would have been an impressive aerial series.

Esposito had a big wobble, and Romania's Sabrina Maneca-Voinea fell twice.

D'Amato had a couple of minor wobbles in an otherwise impressive routine to seize first place just before Biles competed.

The US superstar, again greeted rapturously by Bercy Arena fans, made a confident start, but she somehow went off-line on an aerial series and slipped off, drawing a collective gasp from the crowd.

Visibly disappointed, Biles had to wait an agonizingly long time for her score of 13.100 to be posted.

That was nowhere good enough, and Biles's chance to join Soviet gymnast Larisa Latynina and US swimmer Katie Ledecky as the only women to pile up nine Olympic golds was over -- at least for this year.

The 27-year-old hasn't quite ruled out a return for the 2028 Games in Los Angeles.

In other finals, China's Zou Jingyuan retained his parallel bars title in emphatic style, adding a first Paris gold to his team and rings silvers.

Zou, a three-time parallel bars world champion, delivered a near-flawless performance for a monster score of 16.200 points, becoming just the second back-to-back Olympic champion on the apparatus.

Ukraine's Illia Kovtun took silver with 15.500 points and Japan's Shinnosuke Oka added bronze to go with his team and all-around gold.