Muirhead's Scottish rink defeated Canada and Rachel Homan 6-4 in their opening encounter on Saturday but were no match for Swedish skip Margaretha Sigfridsson second time out.
Sweden scored three points in the first end when fourth player Maria Prytz drew into a busy house before following that up with a promote take-out of a solitary Scottish stone in the fourth.
That gave Sweden five points with the contest over in the sixth end with the score at 11-2 as Sigfridsson exacted revenge on Muirhead for defeat in page one-two play-off at the Europeans.
Sigfridsson fell 9-3 to Muirhead on home ice in December but recovered to take bronze as Scotland settled for silver and she admitted to enjoying every moment of their second successive win in Riga.
"We got off to a good start of course, and then we played better than they did. They had a few more good chances to score but we sorted out it," said Sigfridsson.
"We usually like to play Scotland in the round-robin because we usually win and it was a great win."
Elsewhere Switzerland remain undefeated from their three games after an 8-5 victory over China, who in comparison are yet to record a win from the same amount of matches.
Canada skip Homan bounced back from defeat to Muirhead's Scottish rink to inflict a second defeat from two onto hosts Latvia, running out 7-5 winners.
Russia have two wins and one loss after a 9-4 victory over Italy, who are winless from their three games so far, while Germany are two from two after a 6-5 defeat over USA.
And Japan bounced back from an opening defeat to Switzerland to beat Denmark 6-4 and pull level with Canada, Scotland, USA and Danish skipper Lene Nielsen on one win and one loss.
© Sportsbeat 2013