Few knew quite what to expect as Mollie O’Callaghan waited behind the starting blocks.

She was about to dive into the unknown, because the Duel in the Pool had been lost to Australian swimming for 15 years. Athletes were scrambling to find their names on event schedules to see when they were due in the water to compete for points. Even organisers were trying to wrap their heads around the format long after the races had started.

“There’s less pressure on it because you don’t know what’s happening,” Australian freestyle swimmer Meg Harris said. Some understatement. O’Callaghan said rule explanations were going in one ear and out the other. She reckons even Emma McKeon was “clueless”.

Yet as soon as Australian swimming’s fastest rising star hit the water, the Sydney Olympic Park Aquatic Centre erupted. At its core, the Duel in the Pool between Australia and the United States is a revival of the sport’s great rivalry.

O’Callaghan surged home to clinch the opening event on day two, leading an Australian team boasting Emma McKeon, Zac Stubblety-Cook and Brad Woodward to success in the mixed 4x100 medley relay in front of 3355 fans on Saturday night. It sets up a tantalising finish on day three with the US taking a 159-148 lead.

“It’s pretty sick,” O’Callaghan said. “It’s nice to come back after a big few months, it’s great to do it alongside these guys and especially in front of this big, humungous crowd.”

It was as big as it was loud. Fans threw swimming caps and posters down to McKeon as she stood poolside, hunting autographs from Australia’s greatest Olympian who, like her teammates, relished the chance to compete without the pressure of a major international meeting.

Swimmers were here to win, no doubt, but enjoying a meeting like no other was at the top of the agenda as Swimming Australia looks to build on a bumper 12 months for the sport and engage a growing fan base.