After a confident victory over winless China, the former world silver medallist took an early initiative against Japan, banking four points in the third end on route to an 8-3 victory secured in the eighth end.
"We came out a lot sharper there than in every other game," said Muirhead. "That's what we needed to do.
"We won the draw shot, put a two up on the scoreboard in the first end, forced the one, and then came away with that big four-ender in the third."
The result leaves Scotland well placed in the rankings with just one defeat in five matches with Sweden, the only team to beat them, boasting the only unbeaten record in the 12-team tournament.
Muirhead's rink, which includes Anna Sloan, Vicki Adams and Claire Hamilton, will face Switzerland and winless hosts Latvia on Tuesday.
Final session results
Scotland 8-3 Japan
USA 5-4 Canada (extra end)
Denmark 4-5 Sweden (extra end)
Latvia 3-6 Germany