'Unreal': Aussies shock swimming championships with dual world records

Australia has shocked the swimming world with two incredible world records in South Korea.

The return of Katie Ledecky could not stop Australia upstaging the United States and claiming 4x200m freestyle relay gold in a new world record at the world swimming titles on Thursday night.

Ariarne Titmus, Madi Wilson, Brianna Throssell and Emma McKeon combined to claim victory in seven minutes, 41.50 seconds.

It eclipsed China's 2009 supersuit world record mark by 0.58 of a second.

Anchor McKeon trailed at the last change by 0.09 of a second but overpowered Katie McLaughlin to ensure Australia won by 0.37 of a second and seal their fourth gold of the meet.

It marked the first time Australia had claimed the women's 4x200m relay world title.

"I wasn't thinking about a world record. Was it a 2009 record? To break that this year is so exciting," McKeon said.

"I was hurting a lot but when it comes to a relay you really give it your all."

Olympic champs the US looked set to extend their run of three straight world titles in the event when they welcomed back five-time Olympic champion Ledecky from her sick bed.

Ledecky - a 14-time world champion - had not been sighted in the pool since Monday morning's 1500m freestyle heats, before she succumbed to a stomach bug.

She missed the 1500m final and pulled out of the 200m freestyle altogether.

Ledecky helped the US (7:41.87) also finish under the old record mark but there was no stopping Australia.

"Just touching the wall and seeing the three others celebrate, it made me so excited," McKeon said.

Aussies break two world records

Remarkably it was the second world record broken by the Australian team on Thursday night after Matthew Wilson equalled the 200m breaststroke mark in the semi-finals.

He clocked two minutes, 06.67 seconds, equalling the 2017 world mark set by Japan's Ippei Watanabe.

"I was just trying to get my hand on the wall and get in the final," Wilson said.

"Anything after that was a bonus. I mean a world record is a pretty big bonus.

"I'll just go back and I'll probably watch the footy when I get back, the Sharks are playing, that's my team.

"They weren't going very well last time I checked but I'll do that and it'll take my mind off the swimming."

Earlier, Australia's Olympic champion Kyle Chalmers was relegated to 100m freestyle silver by American superstar Caeleb Dressel.

Dressel clocked 46.96 seconds - just 0.05 off the world mark - to defend his world title.

It was the fastest time ever in a textile suit.

Brazilian Cesar Cielo's 2009 world record of 46.91 was completed in a now banned supersuit.

Chalmers claimed silver in a personal best 47.08 - ranking him third in Australia's all-time list.

In the women's 100m freestyle semi-finals, former world champion Cate Campbell cruised to be second fastest qualifier for Friday's medal race with Emma McKeon third behind Sweden's Sarah Sjostrom.

Kaylee McKeown was fourth in the 50m backstroke final behind US winner Olivia Smoliga.

And Mitch Larkin faded to seventh in the 200m IM final, with Japan's Daiya Seto claiming gold.