The Australian double Olympic champion clocked 3min 55.38sec, breaking the previous record of 3:56.08 set by Canadian teenage sensation Summer McIntosh four months earlier.

McIntosh, who had taken the record off Titmus, took an early lead only for her Australian rival to come roaring back.

Titmus went under 29 seconds for the final 50 metres as she beat American seven-time Olympic champion Katie Ledecky by 3.35 at the Marine Messe Fukuoka in the Japanese city.

"I was really excited for this race coming in tonight, I haven't really raced the best in the world since the Olympics," said Titmus.

"Katie and Summer always put on a great fight and I'm glad we could put on a show."

Ledecky finished in 3:58.73 for silver as she picked up her 19th world medal.

"I can’t really complain," said the 26-year-old.

"I executed the race the way I wanted to.

"I thought Ariarne and Summer would be out fast so I just wanted to stay in contact and go from there."

New Zealand’s Erika Fairweather clinched her first long-course world medal with bronze after touching the wall in 3:59.59, edging McIntosh out of the podium places.

Sam Short delivered another gold for Australia when he came out on top in the men’s 400m freestyle final.

The 19-year-old overcame reigning Olympic champion Ahmed Hafnaoui of Tunisia to triumph in 3:40.68.

Hafnaoui broke the Asian record with 3:40.70 but it was not enough to take gold as he had to settle for silver.

The bronze medal went to Germany’s Lukas Märtens who finished in 3:42.20.

Australia’s third gold of the day came when Jack Cartwright, Flynn Southam, Kai Taylor and Kyle Chalmers clinched the men’s 4x100m freestyle relay title.

The quartet clocked 3:10.16 to seal top spot ahead of Italy who claimed silver in 3:10.49, while the US touched the wall in 3:10.81 for bronze.

Gold number four for Australia was won by their women’s 4x100m freestyle relay team in stunning fashion.

Mollie O’Callaghan, Shayna Jack, Meg Harris and Emma McKeon notched a world-record time of 3:27.96 to take gold, while the US earned silver in 3:31.93 and China secured bronze in 3:32.40.

History was also made in the men’s 400m individual medley as France’s Léon Marchand took American great Michael Phelps’ last remaining individual world record.

Phelps’ time of 4:03.84 had stood as the world’s best mark in the discipline for 15 years after he set it at the Beijing 2008 Olympics.

But the 23-time Olympic champion’s record has now gone after Marchand lowered the mark by an impressive 1.34.

Marchand powered to victory in 4:02.50, sealing his third world title.

"That was insane, one of the most painful things I've done," said the 21-year-old.

"It was amazing to do it here and the time is crazy."

Marchand was in a league of his own as American Carson Foster finished four seconds back for silver in 4:06.56.

Japan’s Daiya Seto completed the podium as he registered a time of 4:09.41 for bronze.

The quarter-final line-up has been set in the men’s water polo competition following the completion of the playoffs.

France have set up a last-eight meeting with Spain after beating Australia 11-8 at Marine Messe Fukuoka.

Two-time Olympic champions Serbia overcame hosts Japan 16-10 and will now face Italy.

The other two playoff matches saw Montenegro edge Croatia 13-12 and the United States defeat Canada 13-10.

The US will next face Hungary, while Montenegro have been awarded with a clash with Greece in the other quarter-final match.