Sigfridsson moved into a first-place tie with Ott after a convincing 7-1 win over Japan's Satsuki Fujisawa in Thursday's seventh draw.
The native of Harnosand scored two in the first, fourth and fifth ends, and one in the sixth, and only allowed a single point in the third.
Meanwhile, Russia's Anna Sidorova fell from the ranks of the unbeaten, falling 6-5 to South Korea's Ji-Sun Kim. Sidorova made it close by scoring one in the seventh end and stealing another in the eighth, but it wasn't enough.
Kim and Sidorova are both tied for third at 2-1.
Saskatoon's Stefanie Lawton (1-1) earned her first win of the tournament, beating reigning world champion Eve Muirhead of Scotland 8-3. Muirhead fell to 1-2.
Ottawa's Rachel Homan improved her record with a 5-3 win over China's Bingyu Wang (1-2).
In the only men's match of the draw, China's Rui Liu beat Tyler George of the United States 7-4.
Liu improved to 2-1 while George fell to 0-2.
There's a four-way tie atop the men's standings with Glenn Howard of Coldwater, Ont., Calgary's Kevin Koe, Winnipeg's Jeff Stoughton and Sven Michel all at 2-0.
Howard, Stoughton and Koe were all scheduled to curl in Thursday's eighth draw, while Michel was to take part in the late ninth draw.