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United States's midfielder Megan Rapinoe (R) celebrates with United States's forward Alex Morgan after scoring during the London 2012 Olympic Games womens semi final football match between USA and Canada at Old Trafford in Manchester, north-west England on August 6, 2012. AFP PHOTO / ANDREW YATESANDREW YATES/AFP/GettyImages

MANCHESTER, England -- Alex Morgan scored on a header in the final minute of the second 15-minute extra period, lifting the U.S. women's soccer team to a dramatic 4-3 victory over Canada and into its fifth straight Olympic gold-medal game.

Morgan, the former Cal star, hadn't scored since the American's Olympic-opening win over France -- a span of 391 minutes -- but she converted in the 123rd minute at Old Trafford, home of the famed Manchester United club.

"I'm still shock," Morgan said afterward. "I've said all along, I don't care who scores as long as we win."

"I'm really happy that Alex Morgan's on my team," teammate Abby Wambach said. "I told you, she'd score a goal that would make everyone shut up, and I think she did."

The Americans will play in Thursday's gold-medal match at London's Wembley Stadium against Japan, which beat France 2-1. Japan beat the Americans in last year's World Cup final.

"This is what we're about," Wambach said. "This is what we're been working for since we lost the World Cup finale to Japan."

The Americans trailed by a goal three times before Wambach tied it for the last time by converting a controversial penalty kick in the 80th minute.

The action was aggressive and ferocious through two 15-minute overtime periods. Wambach nearly converted a head shot with about 90 seconds left in the second overtime.

Megan Rapinoe scored twice in regulation for the Americans, hitting the net in the 54th and 70th minutes.

Christine Sinclair, who played at the University of Portland, delivered a hat trick for Canada, putting her team ahead in the 22nd, 67th and 77th minutes. She became only the second player to score a hat trick against the U.S. women.

"It was a crazy battle -- credit to Canada. I couldn't believe them," Morgan said. "After we scored, they scored."

The U.S. women hadn't lost to Canada in 11 years, going 22-0-4 in matchups with their northern neighbor over that span.

The Americans had won all four of their previous Olympic semifinal games, two of them in extra time, on their way to winning three of four Olympic gold medals. Morgan's goal prevented the first-ever Olympic women's soccer game settled by penalty kicks.

The game was the 500th in U.S, women's soccer history, dating back to 1985. The Americans' first-ever victory, in 1986, was a 2-0 verdict over Canada.