World time-trial champion Caroline Buchanan of Australia was the fastest qualifier as the BMX competition began at the London 2012 Olympic Velopark.
Buchanan finished in 38.433 seconds, 0.210secs ahead of New Zealand's Sarah Walker (38.644), with Mariana Pajon of Colombia third in 38.787.
Great Britain's three-time world champion Shanaze Reade was fifth in 39.368 and world champion Magalie Pottier of France seventh in 39.778 in the event, which determined seedings for Friday's semi-finals.
The riders hurtled down the start ramp one at a time in a bid to claim a strong ranking for the latter stages of the competition, when eight riders will vie for position on an undulating course, full of jumps, tight turns and a tunnel.
Reade was given a rousing ovation and carried the host's hopes.
The 23-year-old from Crewe crashed out of the 2008 Olympic final in Beijing attempting a gold-medal-or-bust manoeuvre and was among the favourites.
Reade was third of the 16-strong field to take to the route, which was modified after she won the test event last summer, and was the provisional leader for a short time, only to see fifth rider Walker better the mark before penultimate rider Buchanan went quicker still.
Pottier, who won the world title in Birmingham in May, was last to start and qualified in a lowly position.
Reade opted not to talk and was set to take tomorrow off before resuming her bid for Olympic glory.