Australian swimmer Sam Short has dedicated his 400m freestyle silver medal to his late auntie after tapping into her fight at the world championships.

The 21-year-old, the 2023 world champion who finished fourth at the Paris Olympics, couldn’t hide his disappointment upon touching the wall.

He had lost by just two hundredths of a second.

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Short overcame an early deficit to hit the lead with less than half the race remaining and still led by a fingernail with 50m to go.

Neck-and-neck with Lukas Maertens right until the end, it was the Olympic champion Maertens who came up trumps.

Maertens posted 3:42.35 to narrowly beat Short (3:42.37) while South Korea’s Kim Woomin (3:42.60) finished third.

Afterwards Short revealed the grief he had been carrying in the lead-up to the world championships in Singapore.

“I won two years ago by 0.02 and today I just lost by 0.02,” Short said.

“I’m happy to be back on the podium after a hard last year. So, you know, I can’t complain.

“I just want to dedicate that performance there to my auntie who passed away a couple weeks ago.

“As bad I was hurting there, it’s nowhere near as bad as her battling cancer for 10 years. So I had to toughen up and get it done.”

In the stacked women’s 400m freestyle field, Lani Pallister produced a personal-best 3:58.87 to finish fourth.

Canadian world record holder Summer McIntosh (3:56.26) blitzed the field to win gold and China’s Li Bingjie surged late to finish second, while the legendary Katie Ledecky had to be content with bronze.

Australia’s 400m Olympic champion Ariarne Titmus didn’t feature after extended her post-Paris break from the pool.

The relay events proved to be Australia’s saviour on the opening night of the championships, propelling the nation to the top of the medal standings.

First, Olivia Wunsch pulled off a huge late comeback to fire Australia to an upset victory over the US in the women’s 4x100m freestyle relay.

Then Kyle Chalmers achieved the same result for Australia’s men, reeling in a sizeable lead from the US to snare gold.

The Australian coaching staff celebrated wildly upon each victory.

Australia entered Sunday night’s 4x100m women’s relay with a new-look team featuring Mollie O’Callaghan, Meg Harris, Milla Jansen and Wunsch.

The US were hot favourites to win, but Australia threw a spanner into the works by clawing their way into the lead by the time Wunsch dived into the water for the final leg.

American Torri Huske quickly re-took the lead and held a half-length margin with 50m remaining before Woods came storming home to snatch victory.

“I definitely can’t believe it,” Jansen told Nine.

“This is something that I wanted growing up. I was feeling very nervous coming into this, but it’s just a dream come true.”

Australia finished in a time of 3:30.60, with the US (3:31.04) in second.

The Australian men followed suit, with the quartet of Chalmers, Flynn Southam, Kai Taylor and Maximillian Giuliani posting a world championship-record time of 3:08.97 to defeat Italy (3:09.58) and the fading US (3:09.64).

“I’ve kind of lost my voice already from cheering so hard,” Southam said.

“I’m just so happy to be here, and we all swam out of our skin, and we did it for the country, and we’re so proud of ourselves.”

- with 7NEWS