USA 14 IRELAND 5, Gorodok Stadium, Moscow
Scorers: USA: Tries: Nathalie Marchino, Victoria Folayan; Cons: Kimber Rozier 2
Ireland: Try: Amy Davis
The USA's comeback win has earned them a crack at New Zealand, the team to beat this weekend, in the Cup semi-finals, while Ireland will clash with arch rivals England in a mouth-watering Plate semi-final (kick-off 3.14pm local time/12.14pm Irish time).
Amy Davis came in as the only change to the Ireland team that overcame China in their final pool game, with Jeannette Feighery dropping down to the bench.
The Americans built some early momentum in humid conditions before Ireland seized on a loose ball to work an overlap - Claire Molloy, Lynne Cantwell and Jenny Murphy did really well to set it up - and Davis, who has played on the wing and at scrum half for the 15-a-side team, flew up the touchline to score in the right corner.
Murphy's conversion attempt came back off the post, but Ireland maintained their lead up to half-time with some smart defending and first-up tackling.
Jon Skurr's side kept working hard and only some committed tackling from the Americans and a subsequent knock on prevented them from going close to a second try.
Ireland then came under further pressure with Alison Miller doing well to thwart the dangerous Victoria Folayan in a foot race and Kelly Griffin, who thought she was through for a try, was called back for an earlier infringement.
Good body positioning at rucks and a high defensive line helped Ireland hold onto their 5-0 lead. Suddenly, they were less than two minutes away from a much coverted semi-final berth.
But the USA's persistence eventually paid off as good work from Folayan set up a levelling try for substitute Nathalie Marchino and the resulting conversion from Kimber Rozier put the Eagles ahead.
They crafted a second try to make certain of their progress into the last-four as Folayan slipped through a tackle and scored under the posts. Rozier tagged on the extras for a nine-point winning margin.
Ireland's cause was not helped by Murphy hobbling off in a tightly-contested second half and head coach Skurr referred to the injury toll - Niamh Briggs and Claire Keohane were among five players who were injured for the tournament - when speaking afterwards.
"The girls were feeling it today. We have had a lot of injuries and we missed a few girls but our fight and determination and commitment has been amazing," he said.
"We have so much potential. We have got a long route ahead of us and we have learned a lot of lessons.
"We're confident that we will get better. I want to thank the girls who are not here as well and the contribution they gave all season long."
The girls in green still have the Plate title to fight for in Moscow, while their superb achievement in finishing in the top eight will see them compete as a core team in the 2013/14 IRB Women's Sevens World Series.
Click here to view the USA v Ireland match in full courtesy of USA Rugby.
Click here to visit the official IRB tournament website for the latest results and updates.
TIME LINE: 3 minutes - Ireland try: Amy Davis - 0-5; conversion: missed by Jenny Murphy - 0-5; Half-time - USA 0 Ireland 5; 14 mins - USA try: Nathalie Marchino - 5-5; conversion: Kimber Rozier - 7-5; 14+2 mins - USA try: Victoria Folayan - 12-5; conversion: Kimber Rozier - 14-5; Full-time - USA 14 Ireland 5
USA: Jillion Potter, Vanesha McGee, Kimber Rozier, Christy Ringgenberg, Victoria Folayan, Irene Gardner, Ryan Carlyle.
Subs: Kelly Griffin, Deven Owsiany, Kathryn Johnson, Emilie Bydwell, Nathalie Marchino.
IRELAND: Jenny Murphy, Claire Molloy, Lynne Cantwell, Alison Miller, Shannon Houston, Amy Davis, Ashleigh Baxter.
Subs: Sophie Spence, Jeannette Feighery, Larissa Muldoon, Laura O'Mahony, Nikki Caughey.
Referee: Amy Perrett (Australia)