In a repeat of the 2008 final, USA - written off by many in the build-up to this tournament owing to a much-changed squad - came from a set down to leave rivals Brazil scratching their heads about how to stem the tide.
But they could not figure it out in time and, with both sides having started with 100 per cent records, Brazil were the ones to leave the court with a blemish against their name.
Captain Clay Stanley was magnificent for USA, spiking 16 times as part of his 19-point haul. On the other side of the net, Brazil failed to get a performance from their leader Giba, who contributed just two points as he was reduced to a bit-part role.
Despite the loss, Brazil remain second in the Pool B table with two games to go and are still favoured to make the last eight, while the USA are already there and will be looking to claim top billing ahead of the knockout stages.
Earlier in the day, Germany came back from two sets down to beat Serbia 3-2 in one of the most exciting games of the tournament so far.
Vital Heynen's squad defied the odds to record their first victory of the tournament, with the full scoreline 22-25, 27-29, 25-18, 25-20, 20-18.
They remain outside the qualification places but now have a realistic chance of going through.
They left the court indebted to the work of the brilliant Gyorgy Grozer. He picked up a tournament-high 39 and, in the fifth set, Germany's plan was simply to tee him up to spike.
Runaway Pool A leaders and surprise package Bulgaria booked their quarter-final spot with a three-set win over Australia at Earls Court.
Although ranked ninth in the world, the Bulgarians have surprised many people with their performances so far, with this victory over a tricky Australian side another notable performance.
They found a way to stifle the Volleyroos' power-hitters - the excellent Thomas Edgar apart - and, with a quite brilliant block at the middle from the likes of Todor Aleksiev, they always had enough to get by.
Russia's men swept past Tunisia in the afternoon to boost their campaign.
The World Cup holders secured their second win in three attempts in a tough pool with Maxim Mikhaylov at the base of a clinical Russian performance, top-scoring with 15 points - two of them on serve.
Poland recovered from their surprise defeat to Bulgaria by whitewashing Argentina.
The World League champions have been heavily backed for success at these Games, but defeat to their European rivals on Tuesday put that under some question.
They responded with a routine win today, though - one which edges them closer to securing a place in the last eight.
In the final game of the night, Italy sent Great Britain to a third successive straight-sets defeat, with the number four-ranked side too cute, strong and tall for their hosts.