The Netherlands' Raymon van der Biezen and Australia's Caroline Buchanan recorded the fastest times in BMX seeding runs Wednesday.
The time trials, one lap around the Olympic BMX track, determine seeding and gate selection for bracket competition. The field of 32 men starts with quarterfinals on Thursday, while 16 women go directly to semifinals Friday.
Van der Biezen set the fastest time among men, clocking a lap of 37.779 seconds. His countryman, Twan van Gendt, was 0.560 seconds behind in third and American Connor Fields was fourth.
Van Gendt will be in a quarterfinal heat with Australia's Sam Willoughby, who won the world championship in May and had the sixth-fastest time Wednesday. Also in that heat is Latvia's Maris Strombergs, who won the first men's Olympic BMX gold four years ago in Beijing.
The United States' Nicholas Long had the seventh-best time Wednesday.
Buchanan's lap of 38.434 was quickest in the women's seeding runs. She will be in a semifinal heat with France's Laetitia Le Corguille, the 2008 silver medalist who was fourth-fastest Wednesday.
Also in that heat will be two Americans. Alise Post was eighth-quickest while Brooke Crain came in last after crashing.
Reigning world champion Magalie Pottier, also from France, had the seventh- best time.