Seavey had reached Koyuk 96 minutes ahead of Zirkle and took about a 5-hour break there while Zirkle rested for about 4 hours, the Anchorage Daily News reported.
The leader shed a dog from his team, leaving him with 10 to pull his sled the final 171 miles to Nome. Zirkle was running with a dozen dogs.
Also in the hunt to win the grueling 975-mile race from Willow, near Anchorage, to Nome were Aaron Burmeister, John Baker, Pete Kaiser, Mitch Seavey and Ramey Smyth, the newspaper said.
From Koyuk, the mushers travel 48 miles to Elim, then another 46 miles to White Mountain where they will take a mandatory 8-hour break before embarking on the final 77 miles to Nome.