Muirhead was victorious in both her matches on Monday against China and Japan, taking her record to four wins and one loss - and she defeated Switzerland skip Silvana Tirinzoni 11-4 on Tuesday.
Scotland scored two in the first end while in the ninth Muirhead had a hit and stay for five after which Switzerland, who had earlier beaten USA 7-6, conceded the game.
Muirhead had extra reason to celebrate as in the same afternoon session Swedish skip Sigfridsson fell to her first defeat at the hands of a resurgent Chinese rink.
Sigfridsson, who beat Muirhead 11-2 on Sunday, was defeated 6-4 by Wang Bingyu, who went into the clash having won her first match of tournament 7-5 against Germany.
Those results leave Scotland and Sweden tied at the top on five wins and one loss and Muirhead is calling for more of the same with five more round-robin matches left in Riga.
"That was really solid out there. We came out sharp, won the draw shot, which is always our first goal in the game, and then got two early," said Muirhead.
"We had a few that slipped back in the fourth end and that cost us the steal, but we had a good second half to finish it off.
"I think it's about consistency now. All the girls are playing great. Our drawing's been pretty good. If you've got the weight, and the kind of game I'm calling, an aggressive game, it's going to be tough for a lot of teams to beat us."
Russian skip Anna Sidorova, who beat Muirhead in the final of the European Championships in December, moved behind Scotland and Sweden with two wins in the first two sessions.
Her rink beat Denmark 5-4 in the morning and then hosts Latvia 9-7 in the afternoon, thanks mainly to a score of three in the ninth end, for a record of five wins and two losses.
Italy suffered the opposite fate as they went down 6-4 to Japan and then 7-6 to Canada with both victorious nations now on four wins and two losses.