The Scottish skip and Swedish rival Margaretha Sigfridsson have both now won eight of their nine games, meaning they will progress from the round-robin stages whatever the result of their final two matches.
Italian skip Diana Gaspari was brushed aside by Muirhead's rink, who secured their seventh straight win with an 8-4 victory.
"It was solid out there and that's guaranteed us a play-off slot," said Muirhead, who faces Russia and the USA in Thursday's concluding group games.
"That was our first goal, so I'm really chuffed. Now we've just got to concentrate, we want to get into the right play-off, the one-two play-off."