The powerhouse United States dominated three years ago in Tokyo, winning 30 medals, including 11 gold, ahead of Australia.
But the Australians turned the tables at the 2023 world championships, winning more titles than their arch-rivals to set the scene for a titanic Olympic showdown.
AFP Sport looks at five blockbuster races:
Women's 400m freestyle
The world record should be in play in one of the most anticipated clashes of the Games, with three swimmers at the height of their powers in the hunt.
Australia's defending champion Ariarne Titmus will dive in as favourite after clocking the second-fastest time ever last month behind only her own 3min 55.38sec world best.
She stunned Katie Ledecky in an electric Tokyo final three years ago, with the American great gunning for revenge.
Ledecky's best time this year is nearly three seconds slower than Titmus, but she can never be written off.
Canadian sensation Summer McIntosh, also a former world record holder, completes the hotly favoured trio, but she too will have to up her game to catch Titmus.
Men's 100m freestyle
American superstar Caeleb Dressel only finished third at the US trials and will not be defending his title, throwing the race wide open.
Australian veteran Kyle Chalmers, the 2016 gold medallist who came second behind Dressel in Tokyo, has qualified, but will have to produce something special to fend off the next generation led by Chinese teenager Pan Zhanle.
Pan stunned the sport with a new world record (46.80sec) in February, with Romanian ex-world record holder and fellow 19-year-old David Popovici the only other man under 47 seconds this year.
American pair Chris Guiliano and Jack Alexy and French duo Damien Joly and Maxime Grousset will be looking to crash the party.
Women's 100m backstroke
The world record has switched in recent years between American Regan Smith and Australian Kaylee McKeown.
Smith lowered the mark in 2019 before McKeown bettered it in 2021 and again last year.
McKeown fired a warning shot with the second-quickest time in history last month, only for Smith to sensationally respond by blitzing the world record again in 57.13 at the US trials.
That time could easily be lowered again as the pair battle for supremacy, with Canada's Kylie Masse and America's Katharine Berkoff likely contesting the minor places.
Smith and McKeown will also be red-hot favourites in the 200m backstroke.
Men's 100m breaststroke
British two-time Olympic champion Adam Peaty is looking to join Michael Phelps, who is currently the only man to win the same event at three, or more, consecutive Games.
The only person to ever breach 57 seconds, the 29-year-old took time out in 2023 to deal with mental health issues before storming back with the year's fastest time in April.
But there is a crowded field gunning to dethrone him, led by China's Qin Haiyang, who won all three breaststroke events at the 2023 world championships, where he set a new 200m world record.
Qin is reportedly one of 23 Chinese swimmers implicated in a major doping scandal.
American veteran Nic Fink and Dutchman Arno Kamminga have both been in good recent form and could pose a threat.
Men's 400m medley
Leon Marchand is France's big home hope and nowhere more so than in the 400m medley after his sizzling 4:02.50 last year shattered Michael Phelps' 15-year-old world record.
The 22-year-old, trained by Phelps' old coach Bob Bowman, is under huge pressure to live up to expectations and must raise his game after failing to go under 4:10 this year as a number of others have done so.
Key competition comes from Americans Carson Foster and Chase Kalisz, Britain's Max Lichfield and New Zealand's Lewis Clareburt.
A five-time world champion, Marchard will also be heavy favourite in the 200m medley and 200m butterfly as he bids to cement a burgeoning reputation.
