Alberta's Chelsea Carey edged Northern Ontario 9-8 in the afternoon and then beat Prince Edward Island's Suzanne Birt 5-4 to remain the only undefeated team at 5-0.
"It's a good thing to have to scratch and claw and make some big shots and have the other team outplay you and have to come back from that," Carey said. "You need to be game-tough to survive a week like this.
"Probably not going undefeated. Eventually you probably lose a game and you know that. You prepare for it. It's not a big deal when it happens, but nice for it to not happen yet."
Northern Ontario fell to 3-1, but has forged a reputation for tenacity among the 12 teams in Grande Prairie, Alta.
The Fort William Curling Club team trailed Alberta by four points after the fourth and eighth ends, but forced Carey to throw her final stone in the 10th to avoid an extra end.
The previous day, Northern Ontario stole two points in the 10th and another deuce in an extra end to get by Nova Scotia. Krista McCarville, Northern Ontario's skip, also made a pressure draw to the four-foot rings to score two for a victory against defending champion Jennifer Jones.
"We have to take advantage of being out here," the skip said. "I'm sure there's millions of teams that would love to be out here. We can't get down on ourselves. We're still playing pretty good."
Saskatchewan's Jolene Campbell and Nova Scotia's Jill Brothers both won Monday to get to 3-2. Jones also won to pull even with Quebec's Marie-France Larouche, Manitoba's Kerri Einarson and P.E.I. at 2-2.
Ontario's Jenn Hanna, New Brunswick's Sylvie Robichaud and B.C.'s Karla Thompson were all 1-3 and Stacie Curtis of Newfoundland and Labrador was 1-4.
The top four teams at the conclusion of the preliminary round Friday advance to the Page playoff. Ties for fourth are solved by tiebreaker games.
The winner of Sunday's final represents Canada at the women's world curling championship March 19-27 in Swift Current, Sask., and wears the Maple Leaf at next year's Tournament of Hearts in St. Catharines, Ont.