The Olympic races are held over a course of just under a quarter of a mile with the motorcycle riders starting at the top of a 25-foot ramp.
Eight riders in each quarterfinal heat competed three times and Day won all three of the races in his heat for 3 points. Marc Willers of New Zealand finished fourth, third and second in his three races and his nine points was good for a second qualifying position.
Sifiso Nhlapo of South Africa won the second heat, Maris Strombergs of Latvia took the third and Jared Graves of Australia the fourth. Day was the only rider to win all three races in his heat.
A competition is also being held for women but only 16 competitors are entered.
Gold medals will be awarded Thursday in both the men's and women's events.