Australia has produced a stunning one-two punch on the opening night of the world championships to stun their highly-favoured rivals.

Cop that America, Australia have claimed the 4x100m relay double at the Singapore World Swimming Championships on Sunday.

First up it was the women who locked horns with the Aussie quartet of Mollie O’Callaghan, Meg Harris, Milla Jansen and Olivia Wunsch taking to the pool.

With two teenagers in Jansen and Wunsch in the team, America went into the final as the favourites but the stage didn’t overawe the youngsters.

After O’Callaghan and Harris got the Aussies off and running, it was the two youngsters who brought it home.

In what was a neck and neck battle, it was Wunsch who produced the swim of her life to hunt down American Tori Huske and touch the wall first by only 0.44 seconds.

Mollie O’Callaghan, Meg Harris and Milla Jansen celebrate after winning gold. (Photo by Adam Pretty/Getty Images)

Then it was over to the men who entered the final as heavy underdogs against the strength of the Italian and American squads.

Flynn Southam, Kai James Taylor and Maximillian Giuliani gave it their all through the first three legs, but the Aussies were still stranded behind the Americans.

Enter Kyle Chalmers.

The sprint king produced a final leg for the ages as he hunted down his Italian and American rivals to touch the wall and hand Australia a commonwealth record and the gold medal.

Flynn Southam, Kai James Taylor and Maximillan Giuliani cheer Kyle Chalmers home. (Photo by Maddie Meyer/Getty Images)

Chalmers, like the rest of us, couldn’t believe what he’d done. (Photo by Sarah Stier/Getty Images)

Earlier, Germany’s Olympic champion Lukas Maertens won a thrilling 400m freestyle gold pipping Australia’s Sam Short by 0.02sec. in a nailbiting finish.

Maertens, who broke the world record earlier this year, came home in 3min 42.35sec after a fierce battle with Short, the 2023 world champion.

South Korea’s Kim Woo-min, the reigning world champion, was third in 3:42.60. Maertens was the favourite for the title after breaking the world record in Stockholm in April, a mark that had stood since 2009.

But Short fought him every stroke of the way, losing out after an incredible tussle to the finish.

“I thought we were going a bit faster, to be honest,” Short told Channel 9 after the race.

“It’s a high-pressure event. Two fast 400s in a day, really hard. I won two years ago by 0.02, and I just lost by 0.02. I’m happy to be back on the podium after a hard last year, so I can’t complain.

“I didn’t even know how I was going to go. All staging camp, or at least 75 per cent of it, I was in quarantine, I had Covid. A lot of mental strength that I’ve learned this year, I’m just stoked.”

Sam Short fell agonisingly short in the final. Picture: Adam Head

Short later went on to reveal that his aunty had recently passed away, dedicating his performance to her.

“This year has been really hard,” he said.

“No one really knows everything. I want to dedicate that performance to my aunty who just passed away a couple of weeks ago… it’s been quite hard for my family recently… however bad I was hurting there, nowhere near as bad as her battling cancer the last 10 years.”

The result gave Maertens his first world title after claiming Olympic gold in Paris last year.

He followed that up by setting a new world record of 3:39.96, shaving 0.11sec off the mark achieved by fellow German Paul Biedermann at the world championships in Rome in July 2009.

Biedermann’s mark of 3:40.07 was achieved wearing a polyurethane swimsuit that has since been banned in competition.

Short missed out on an Olympic medal in the event in Paris, finishing fourth. Australia’s Elijah Winnington, the Paris Olympics silver medallist, surprisingly failed to qualify for the final.

Short finished fastest in the morning heats ahead of Maertens.

– with AFP