Australia and the United States have added fuel to their simmering rivalry as the top swimming nations at the Tokyo Olympics - but it is the emergence of two teenage sensations that could spark the flames at Paris 2024.
Mollie O'Callaghan is already well down the path to being Australia's next swimming superstar while the USA can look to 17-year-old Bella Sims as their comparable young gun.
The 18-year-old O'Callaghan claimed gold in the 100m freestyle at both the Commonwealth Games and world championships, as well as silver medals in the 200m freestyle at the same two meets.
The Queensland teen also has multiple freestyle relay medals, including two Olympic golds, but missed out on adding another title to that collection when coming up against the USA and Sims in the 4x200m freestyle relay at the world championships.
O'Callaghan won silver in the individual 200m freestyle behind China's Junxuan Yang the day before, so when she entered the pool 1.07 seconds behind Sims in the final leg of the 4x200m a close finish seemed likely.
But Sims powered away from her highly-fancied opponent to stretch the lead to 2.41 seconds and keep Australia in second.
"I think that was the first time I've ever raced her, in that relay," Sims said about the race and O'Callaghan.
"It took all I had. When I got out, I could not walk, I couldn't even really stand my legs were hurting so bad.
"My team wanted to hug but I needed to sit down first."
A much-anticipated rematch was on the cards for the Duel in the Pool event between Australia and the USA at Sydney Olympic Park Aquatic Centre, but Sims and O'Callaghan are being made to wait.
Sims swam in the 4x800m open water relay that opened the event on Friday, before winning the 200m leg on her way to taking out the 400m broken freestyle on Saturday.
The USA swimmer was also narrowly beaten in the traditional 200m freestyle later that night.
Unfortunately for the raucous crowd of 3355, O'Callaghan skipped those races to instead focus on events up to 100m at the meet.
"We have such great depth in 200 and 100 freestyle, so I knew the girls would be perfectly fine without me," O'Callaghan said about missing the rematch with Sims.
"I guess I was needed in the 100 backstroke, so I was happy to do that.
"But it's always good to have competition coming up."
The teams are yet to decide on their swimmers for the races to be held on Sunday, but with the mixed freestyle 'random' relay scheduled to be the last event it could see O'Callaghan and Sims go head-to-head while also determining which nation claims the Duel in the Pool title.
