Adam Ginning, Philip Broberg and Karl Henriksson stood for their goals in the first period.
But in the beginning of the second period, the United States scored four goals in eight minutes and led 5-3 after two periods, with Toronto-drafted Nick Robertson accounting for two of the goals.
Super talent Alexander Holtz reduced for Sweden, where the other great talent, Lucas Raymond, still rested after an infection he suffered.
The United States decided in an empty box.
The junior crowns have not lost a group game match since 2006 and will start this year's tournament against the reigning champions, Finland on Thursday.