The two superpowers have been sizing each other up before the serious action kicks off on Saturday, starting with men’s 400m individual medley heats.
"I briefly saw them this morning, they were warming up as I was swimming and by the time I was hopping out they were hopping in," said Australia's double gold medalist Emily Seebohm.
Team-mate Cameron McEvoy, Australia's big hope of a first 100 freestyle men's gold since 1968, said both sides are familiar with each other.
"A lot of us are really, really good friends with the Americans," he said. "It was exciting to see them coming. They are a powerhouse team and you can definitely feel their presence in the pool.”
USA won 31 medals (16 gold) four years ago in London, while Australia won 10, including one gold. The two powers were more evenly matched at last year's world championships in Russia, with the Americans winning 23 medals and eight golds to Australia's 16 and seven golds.
The rivalry over the years has been immense with the 4x100 freestyle relay at the 2000 Sydney Games providing one of the greatest duels. USA had never been beaten in the event at an Olympics but the Australians broke the world record.
"It's always special to stand up against the Americans," said McEvoy.
"Over the years we've had a really healthy rivalry," agreed 100 freestyle world record holder Cate CAmpbell.
"We've pushed them and they've pushed us. I don't think America would be where they are today without Australia....and America has pushed us to be the great swimming nation that we are as well."