Canada made up the deficit and beat the defending champions 5-4 in a shootout in front of a sellout of 15,003 at the MTS Centre.
Canadian captain Hayley Wickenheiser was the only shooter to score and goaltender Charline Labonte faced down three Americans during extra shots.
Wickenheiser helped spark the comeback with Canada's third goal in regulation time. The Canadian captain also had two assists.
"She'll make me look good if she keeps going like that for the next two games," Canadian head coach Melody Davidson said.
Canada meets Finland on Monday (8:30 p.m. ET).
The Canadians wore pink jerseys Saturday to honour their counterparts who won the first women's world championship in 1990 wearing that colour.
The majority of that 1990 team was on hand for an on-ice reunion between periods. The building was full and the game on national television was a natural tie-in to NHL games with playoff implications televised later Saturday.
For all those reasons, the Canadians were relieved to pull out the win. They also recognized the benefit of playing an exciting game for spectators both in the building and watching on television because women's international hockey is plagued by lopsided scores.
"From our standpoint, it wasn't our best game for sure," Wickenheiser said. "So to come out of it with a win, we'll take it.
"Especially today, with the pink jerseys and the 1990 team in attendance, it was a nice game for us to win for those women that brought the game to where it is today."
Canada also got goals from Kelly Bechard, Sarah Vaillancourt and Gillian Apps during regulation time.
Jenny Potter, Krissy Wendell, Gig Marvin and Natalie Darwitz replied for the U.S.
Labonte stopped 25 shots for the victory. American goaltender Jessie Vetter, playing in her first game against Canada, turned away 26.
The U.S. and Finland meet Sunday.
Canada, the U.S. and Finland finished first in their respective groups. The countries with the best two records in a playoff round robin between the three advance to Tuesday's final (8:30 p.m. ET).
The third-place team drops to the bronze-medal game against the winner of the round between the group runners-up.
Barring a Finnish upset of their next two opponents, Saturday's between Canada and the U.S. was a preview of the final.
After settling for the bronze medal at the Olympics, the U.S. team was revamped. This team has several new, young forwards in their lineup and Mark Johnson was hired as head coach in the off-season.
After twice losing to Canada at the Four Nations Cup in Kitchener, Ont., in November, the Americans felt with their performance that they'd made up ground lost during their rebuilding.
"Most importantly for the newcomers on the team, they got this experience playing in front of 15,000 fans and knowing that Canada isn't this mystical thing and they can be beatable and they do have some weaknesses," Darwitz said. "We're excited about what we did tonight.
"Yeah, it wasn't the outcome that we wanted. But the important thing, it wasn't the championship game. We still have an opportunity to win the tournament and to win the world championship."
Labonte was in net for Canada's last loss to the U.S., which was in Winnipeg on Jan. 1, 2006. She didn't want to drop two in a row to their archrivals in that city.
"You try to forget about that stuff, but at the same time, you come back here to the same place, same crowd, same team and I just wanted to give a better show and give my team a chance to win," Labonte said.
In the shootout, Julie Chu clipped the post behind Labonte, Vetter shut down Canada's Vaillancourt, Labonte denied Darwitz and Wickenheiser scored.
Labonte then made the game-saving stop on Marvin, whom she also denied on a spectacular, sprawling save in the third period after Apps tied the game at 14:57 of the second period.
"Save of the game right there," Wickenheiser said. "I don't think Charlie played her best game either, really, but she made the saves to win the game for us and in the shootout, she shut the door."
Canada had problems working the puck out of its own end in the face of a speedy American forecheck early in the game.
The Canadians also gave up three odd-man rushes in the first period, although those didn't result in goals.
"We gave them a few tonight in terms of easy walks to the net," Wickenheiser said. "We don't want to be giving up four goals against the U.S. because that's one too many."
Canada twice came from behind to tie the game in the first 40 minutes before a scoreless third.
The Americans had 46 seconds worth of a two-man advantage at the end of the third period and Canada had two power plays in overtime, yet neither team converted them to the game winner.
Player of the game - Hayley Wickenheiser. The Canadian captain continues to come through for her team in tight spots. Honourable mention goes to forward Katie Weatherston, who took a lot of punishment as a human bowling ball Saturday. She's an energy player and tried to spark her team when they were down two goals.
Notes - The pink jerseys worn by the Canadian team Saturday are up for auction on ebay to raise money for the Canadian Breast Cancer Foundation and the Hockey Canada Foundation . . . Canada improved to 5-4 all-time against the U.S. in shootouts... Germany beat Kazakhstan 3-0 in relegation round match.