SYDNEY (AFP) - World champion Mollie O'Callaghan was untouchable in the 100-metre freestyle and Emma McKeon too hot over the 100m butterfly, but it was not enough to prevent the United States edging out Australia to claim another "Duel in the Pool" crown on Sunday (Aug 21).

Teenager O'Callaghan burst on to the scene this year, taking both the world and Commonwealth 100m titles among a flurry of medals, and she comfortably won again at the Sydney Aquatic Centre.

The 18-year-old hit the wall in 54.01 seconds ahead of Madi Wilson on the third and final day of a competition centred around traditional sprint events but also never-seen-before formats with points accumulated across individual and relay races.

"After (the) Commonwealth Games, it was hard to get motivated again and train, but the support from everyone on the team has been amazing and I'm really enjoying (the meet)," said O'Callaghan.

The United States went into the final day leading by 13 points and ultimately won the competition 309-283 to keep intact their unbeaten record spanning 19 years and eight Duel in the Pool events.

The event initially took place every two years from 2003 to 2015, with the first three meets against Australia and the next four against Europe.

It underwent a revamp to broaden its appeal for its return, with regular racing joined by other quirky formats designed to add fun and excitement, where unique point bonuses were earned and strategy was crucial.

They included "mystery" medley races in which the order of strokes was randomly determined at the start, meaning some started off the blocks swimming butterfly, while others began with breaststroke, freestyle or even backstroke.

There were also "broken" 400m and 800m freestyles, where swimmers stopped at certain distances to earn points.

The 400m free had a 200m race, followed by a pair of 100m races off the blocks after a brief break, with points awarded to the winner of each.

The meet included a mixed 4x50m relay featuring disabled and non-disabled swimmers, the first time such an event has ever been held, as the sport looks to further integrate para athletes. The US took the inaugural honours.

Among the regular events, McKeon was always in charge in the 100m butterfly, touching more than a second ahead of American Gabi Albiero in 57.05.

"I like knowing there's extra pressure, it makes me race harder," said Olympic star McKeon, who also contested the relay with para athletes.

"I definitely like the newer races, the able-bodied and para one was the first time it's ever been done and so good to be part of."

A strong US team claimed a thrilling men's 4x100m freestyle relay in 3min 14.15sec while American world 50m backstroke champion Justin Ress took the 100m title in 53.12.

American teenager Luke Hobson outpaced teammate Trenton Julian to touch in 1:45.59 and win the 200m freestyle, fulfilling a season ambition.

"I told myself earlier this year I wanted to do a 1:45 and I got it here," said the fast-improving Hobson.

David Curtiss won the men's 50m freestyle for the USA in 21.84 while Australia's Chelsea Hodges claimed the women's 100m breaststroke in 1:06.90.

In Rome, Italy's Domenico Acerenza rammed home the hosts' dominance in the European Swimming Championships by winning two golds on Sunday, a day after his hopes of victory in the 25km race were dashed by the weather.

The 27-year-old, who finished behind compatriot Gregorio Paltrinieri in the 5km open-water race on Saturday, took the honours at the 10km distance.

He overtook two French rivals in the final kilometre to finish in 1:50:33.6 and give Italy yet another gold at the championships.

Marc-Antoine Olivier finished second - adding to his bronze from Saturday's 5km - with Logan Fontaine third.

Acerenza then helped add his country's 24th gold of the event in Rome in the afternoon's mixed 4x1,250m, the Italians finishing in 59:43.1, a full 10 seconds ahead of the Hungarians.

Sunday's haul was rich compensation for Acerenza after a disappointing and frustrating end to his second race on Saturday.

Along with Paltrinieri, he had been in front in the 25km race when both the men's and women's events were abandoned as the weather deteriorated.

No medals were assigned although the swimmers had completed more than 15km of the race. European Aquatics said in a statement that the judges "did not have a view of the final rankings".

Germany's Leonie Beck won a thrilling women's race in 2:01:13.4sec edging out Italy's Ginevra Taddeucci and Angelica Andre of Portugal.