The duo performed to the music of Carlos Santana ("Black Magic Woman" and "Smooth") while representing the Skating Club of Boston.
After skating last of the 12 pairs to compete on Thursday, they finished first with 73.13 points.
A victory would assure them of an Olympic berth in Sochi. They can clinch their second straight national championship on Saturday with another strong performance.
"Thank you, Boston," Shnapir said. "What a crowd. I mean this was just from start to finish such energy, such excitement, such support. We couldn't have done it without our friends and family and all of our supporters and fans here in Boston."Shnapir's family emigrated to the United States when he was 16 months old.
He graduated from Lincoln-Sudbury Regional High School in 2005.