The American phenom, who won team, all-around and vault golds in Paris, is the favourite on floor but not a shoe-in on beam, the apparatus where a mere centimetre of misjudgment can separate a medallist from last place.
If Biles wins both beam and floor, her nine Olympic golds will tie her with U.S. swimmer Katie Ledecky and Soviet gymnast Larisa Latynina for the most ever won by a female athlete.
Her beam challengers, teammate Sunisa Lee and Zhou Yaqin of China, are two of only three women who so far this year have performed a more difficult routine than Biles.
Zhou was the top beam qualifier with a 14.866, just 0.133 ahead of Biles, while third-placed qualifier Rebeca Andrade of Brazil has the lowest difficulty potential of the top four gymnasts but is known for her clean execution.
Only one of Lee’s three beam performances in Paris so far matched Biles’ in difficulty, but the fourth qualifier and all-around bronze medallist edged her star teammate on the apparatus in that competition.
Biles won beam bronze in the last two Olympics, including in Tokyo where she performed a modified routine without twisting, the source of the mental block that forced her to withdraw from most of those Games’ finals.
Biles enjoys a difficulty advantage on the floor exercise with her signature tumbling, including a stratospheric double back flip with three twists, meaning the true race on the apparatus will most likely be for silver and bronze.
Contenders include Andrade, who could be saving difficulty upgrades for the final, Romania's Sabrina Maneca-Voinea, who has performed the second most difficult routine so far in Paris, and American Jordan Chiles who qualified third ahead of Maneca-Voinea.
The male gymnasts conclude Olympic competition on Monday with finals on the parallel bars and horizontal bar.
China’s Zou Jingyuan seeks to retain his Tokyo gold, qualifying in first on parallel bars with a stunning 16.200, almost a point ahead of second-placed compatriot Zhang Boheng.
Zhang led the field on the horizontal bar in the preliminaries with 15.133, 0.533 ahead of fourth qualifier and 2020 silver medallist Tin Srbic of Croatia.
Defending Tokyo horizontal bar champion Daiki Hashimoto will miss the final after errors in qualifying, but his teammate and Paris all-around winner Shinnosuke Oka will be a contender.
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