The Olympic champion and her brakewoman Chelsea Valois had a combined time of 2 minutes, 16.06 seconds for the first two of four runs, 0.12 seconds faster than Germany's Anja Schneiderheinze and Lisette Thoene.
Schneiderheinze piloted her bob to the fastest speed of the day, 142.75 kph (88.7 mph), to close the gap in the second run.
Elana Meyers and Katie Eberling of the United States placed third, with 0.18 to make up in two more runs scheduled Saturday on the Olympia course. Meyers won the bronze last year.
The two-man bobsled competition also begins Saturday.