Her coach may not have been satisfied but Australia's Ariarne Titmus believes she has done enough to start one of the great swimming rivalries after her stunning 400m freestyle world title victory over American great Katie Ledecky.

Titmus became the first Australian woman to claim the world 400m title since Tracey Wickham in 1978 after pulling off a boilover victory that snapped Ledecky's staggering winning streak.

Titmus clocked a Commonwealth record three minutes, 58.76 seconds on the opening night of the eight-day world titles in Gwangju, South Korea, on Sunday to upstage the once untouchable Ledecky.

The American had not been beaten in 400m, 800m or 1500m freestyle at a major international meet since 2012 until the 18-year-old pulled off one of the great boilovers.

Yet Titmus' hard marking coach Dean Boxall still found fault with the landmark swim.

"I think she could have executed it a little bit better. It was a good swim, of course ... but it could have been better," he said.

"I thought she nearly missed an opportunity there."

Still, Titmus was confident she had done enough for Ledecky to take her seriously as a threat ahead of the 2020 Tokyo Olympics.

Ledecky had refused to speak specifically about Titmus ahead of the titles despite the world No.2 ranked teenager's impressive form, stonewalling reporters by claiming "everyone was a threat".

Titmus reckoned that may now have changed.

"Now hopefully she is excited that she now has a battle with me," Titmus said.

However, Boxall warned Titmus' win may come at a price.

He believed a "pissed off" Ledecky would now be out for vengeance in her other two events in South Korea against Titmus - the 200m and 800m freestyle.

"She looks pissed off, angry. She's going to come back," he said of the American.

"She'll be like a caged tiger coming out. Is it great for Arnie to win? Yeah, sure.

"But what does it do for Ledecky? Ledecky is still the Olympic champion, she's the world record holder - Arnie is still the underdog."

However, Australian head coach Jacco Verhaeren believed Titmus had finally reduced world record holder Ledecky to mere mortal status with the victory.

"It shows that she's human and has to work for her results. She's a great competitor, all respect for Katie but for Australia, it's great news," he said.

The usually ever cheerful Ledecky admitted she was hurting from the loss after finishing in 3:59.97 - well off her world record of 3:56.46 set at the 2016 Rio Olympics.

"She ran me down. This stings a little, it's unfamiliar and different," she said.