United States grabbed a last-gasp extra-time winner to claim an enthralling 4-3 victory over Canada and seal their place in the women's Olympics football final for the fifth successive tournament.
Not even a hat-trick from Canada captain Christine Sinclair - the first against USA for 11 years - could deny the Americans their place in Thursday's showdown at Wembley.
That match will now be a repeat of last year's World Cup final in which Japan triumphed on penalties following a 2-2 draw at the end of 120 minutes.
Another shootout was on the cards at Old Trafford until, in the third minute of injury time at the end of extra time, Alex Morgan sent a looping 10-yard header over goalkeeper Erin McLeod to break Canadian hearts.
John Herdman's team took the lead three times, but three times they were pegged back to force the additional 30 minutes.
Japan moved to within one win of completing a Women's World Cup and Olympic double after surviving a late France onslaught to win their semi-final clash 2-1 and reach the London 2012 final.
Last summer's World Cup winners set up a gold medal match against the United States or Canada as they clung on desperately to win their semi-final at Wembley, Elise Bussaglia missing a penalty that would have taken the game to extra-time.
Japan punished woeful set-piece defending to go 2-0 ahead, Sarah Bouhaddi's howler gifting Yuki Ogimi the opening goal and non-existent marking allowing Mizuho Sakaguchi to double the lead.
France then went for broke, substitute Eugenie Le Sommer scoring with 14 minutes remaining and winning a penalty two minutes later, but they were ultimately left to fight it out for a bronze medal shortly before Thursday's final.