Sweden had sensationally put an end to USA's 44-match unbeaten streak at the start of the women's tournament, with Megan Rapinoe admitting the beaten world champion had "played a bit tight" in a 3-0 defeat.
However, Vlatko Andonovski's side responded impressively to that setback, thrashing the Football Ferns 6-1 in Saitama, in the process registering its largest margin of victory in an Olympic Games fixture.
The result will also send shivers through the Matildas camp after Australia lost 4-2 to Sweden, meaning a victory against the USA powerhouse is vital to ensure qualification for the knockout stages.
Rose Lavelle and Lindsey Horan gave USA a 2-0 half-time lead, the latter on target to mark her 100th cap.
Further efforts from Christen Press and Alex Morgan added to the score after the break, while they were helped out by their opponents scoring two own goals.
It is Sweden that sits on top of the table, though, as a strong second-half performance sealed its triumph against Australia, Sam Kerr missing a crucial penalty for the Matildas that would have levelled the match at 3-3 in the second half.
Meanwhile, Barbra Banda tied an Olympic Games record with a second successive hat-trick for Zambia.
Banda equalled the record for most goals scored by a woman at a single Games, matching Christine Sinclair's tally of six for Canada at London 2012 with another treble, this time against China.
Zambia's captain scored all her country's goals in a 10-3 loss to Netherlands in its Group F opener. She followed up with three more in a thrilling 4-4 draw on Saturday, though China's Wang Shuang went one better as she hit four in a see-saw contest.
Vivianne Miedema later moved level with Banda on six goals at this Olympics. The forward had scored four as the Dutch crushed Zambia, then managed another two in a 3-3 draw against Brazil.
Ellen White continued her impressive start to the tournament with the winner as Great Britain defeated host Japan 1-0 to qualify for the last eight.
Having scored twice in the opening 2-0 win over Chile, White made it 13 goals in her past 14 international outings when heading in a cross from Lucy Bronze in the 74th minute.
Also in Group E, Canada overcame Chile 2-1 thanks to a brace from Janine Beckie, putting them second in the standings ahead of a clash with Team GB to finish the round-robin stage.