The United States roared back from a 19-point deficit in three minutes to beat Argentina 21-19 and book a quarter-final showdown with South Africa at the Las Vegas Sevens.
US coach Mike Friday injected Maka Unufe after Argentina had scored their third try and the 25-year-old dotted down twice and assisted Perry Baker's match-winner to keep the Eagles in with a chance at a first victory in the Rugby World Sevens Series.
The triumph followed a stinging defeat in the hosts' final Pool B match, when they ran out to a two-score lead over England only for the second-ranked team in the series to score three tries in as many minutes to steal a win.
"We continued to work for each other and fight for each other," US captain Madison Hughes said. "We didn't give up and believed in ourselves right to the very end."
But they'll have their work cut out against series leaders South Africa, who reached the last four with a 17-14 victory over Australia.
The Australians scored first through James Stannard, before the Blitzboks replied with a try from Ruhan Nel.
Siviwe Soyizwapi put South Africa ahead early in the second half before Liam McNamara pulled Australia level.
Rosco Speckman scored the winning try when he finished down the right wing to propel the Blitzboks, winners of Pool A after a 21-7 win over Wales, back into the lead.
Olympic champions Fiji kept their defense of their USA Sevens crown on course, downing Canada 31-12 to book a semi-final meeting with New Zealand -- 19-0 quarter-final winners over England.
The Fijians, who are still without a tournament win in this season's series, denied Canada with a clinical display.
Apisao Domolailai gave Fiji an early 7-0 lead before speedster Justin Douglas got Canada on the board but tries from Domolailai and Vatemo Ravouvou pushed the Fijian lead to 21-5 at the interval.
Waisea Nacuqu extended the Fiji lead to open the second half before John Moonlight touched down for Canada's second try but a late Samisoni Virivri try propelled Fiji -- unbeaten winners of Pool D -- into the final four.
After settling for second behind Argentina in Pool C, New Zealand took out their frustration on England with two tries from Tim Mikkelson and one from DJ Forbes.