Lacasse led Canada in the pre-Olympic exhibition win, but said her teammates made the busy night easier.
"We kept them to the outside, and then any rebound in front of the net that I let out, our (defense) boxed out and our forwards got the puck out and it was just perfect," she said.
Jillian Saulnier scored the game's first goal with 3:10 left in the third period on a backhand shot past American goalie Alex Rigsby. Laura Fortino added an empty-netter with 44 seconds remaining.
Rigsby made 19 saves in the fourth game of a six-game series leading up to the Winter Olympics on Feb. 9 in Pyeongchang, South Korea. Canada leads the series 3-1.
Canada was 0 for 3 on the power play and the Americans went 0 for 5 in front of a crowd estimated at more than 11,000 at Bell MTS Place.
"You have to score on the power play, there's just no two ways about it," U.S. coach Robb Stauber said. "We had some looks and good chances."
Saulnier called her goal a "full-line" effort, from Brianne Jenner getting the puck in deep and Jennifer Wakefield battling for it with a defender along the boards.
"(The puck) ended up popping out to me and I was lucky to get around the defenseman and back it in with my backhand there," she said.
The Americans led shots on goal 13-7 after the first period and then the pace and physicality picked up in the second, with some roughing and body-checking calls.
American winger Kendall Coyne challenged Lacasse twice, making her whip her glove to grab the puck. Coyne then made a slick move to get around Manitoba defender Halli Krzyzaniak and Lacasse turned aside the shot.
Shots on goal favored the Americans 25-15 after the second.
"We had great chances, we outshot them, but obviously that's not going to be good enough to win," Coyne said. "We've just got to figure out how to get one by whatever goalie's in the net that night."
Canada won 2-1 in overtime on Sunday before a crowd of 9,000 in St. Paul, Minnesota. Both teams are carrying 26 players before the cut to 23 for the Olympic roster on Jan. 1.
The teams face off again Dec. 15 in San Jose, California, followed by the final exhibition Dec. 17 in Edmonton, Alberta.
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