Japan's Seto makes history as three more records fall at short course worlds
Two further world records tumbled on day five in Melbourne -- in the men's and women's mixed 4x50m relays.
Seto, 28, swam a storming breaststroke leg to claim the lead then powered home with his unbeatable freestyle to win the 400m medley in 3mins 55.75secs.
It was the sixth consecutive time he has won gold, a feat never achieved before at either a short course or long course world championships.
His reign began in 2012 and he kept it intact by touching ahead of American Carson Foster by nearly two seconds with South Africa's Matthew Sates earning bronze.
Meilutyte then blitzed the field to set a new breaststroke mark in the semi-finals, hitting wall in 28.37 to lower the 28.56 held by Jamaica's Alia Atkinson since 2018.
She qualified a hot favourite for Sunday's final ahead of American Lilly King.
Australian pool queen Emma McKeon, who dominated the Tokyo Olympics, was also on fire, joining an elite club of just four sprinters to win the women's 50-100m freestyle double.
She was always in charge of the splash and dash to touch in 23.04, with Poland's Katarzyna Wasick left in her wake.
"Honestly, I still don't call myself a 50m specialist, I prefer the 100," said McKeon. "I knew I had to just kill the start, the start is one of my strengths."
In the men's 50m sprint, Jordan Crooks stunned the field clinch the Caymen Islands first medal of any colour at a world championships. He came home in 20.46 ahead of Britain's Ben Proud.
"I just feel blessed, it's an awesome experience," said Crooks.
Day five began with a bang when Australia smashed the 4x50m medley relay world record.
Mollie O'Callaghan, Chelsea Hodges, McKeon and Madi Wilson shaved the 2018 best held by the United States by touching in 1:42.35, with the Americans second and Sweden third.
"I knew we could do something special," said Hodges. "I just love racing in relay, it's so exciting."
Not to be outdone, Italy's Lorenzo Mora, Nicolo Martinenghi, Matteo Rivolta and Leonardo Deplano matched the feat in the men's 4x50m medley.
They hit the wall in 1:29.72 ahead of the United States and Australia to better their own world mark set last year.
They were the eighth and ninth world records shattered so far with one day left.
Distance great Gregorio Paltrinieri backed up his convincing 1,500m freestyle gold with another comfortable victory in the 800m, an event making its debut on the short course program.
The Italian was never troubled, breezing through in 7:29.99 ahead of Norway's Henrik Christiansen and France's Logan Fontaine.
Hali Flickinger won the women's 400m medley for the United States, leading from start to finish in 4:26.51 ahead of Italy's Sara Franceschi and Japan's Waka Kobori.