Australian Mollie O'Callaghan overcame "panic" to continue the surge of teenage medallists in Budapest and become the youngest women's 100m freestyle world champion in more than 30 years on Thursday.

Two American veterans, Lilly King in the women's 200m breaststroke and Ryan Murphy in the men's 200m backstroke, then won breakthrough gold medals before the US men finished the evening by winning the mens 200m freestyle relay.

Australian, Zac Stubblety-Cook, who took the men's 200m breaststroke, said there were reasons this had been a youthful competition.

"The year after the Olympics is a bit of a different feel," the 23-year-old said. "The young bloods are coming through."

He pointed out that in his event "two of us from the Olympics were in the final tonight."

The 18-year-old O'Callaghan, followed 17-year-olds David Popovici of Romania and Benedetta Pilato from Italy and 15-year-old Canadian Summer McIntosh in collecting individual gold at these championships.

O'Callaghan edged the 28-year-old world record holder Sarah Sjostrom of Sweden by 0.13sec. American Torri Huske took bronze.

At 18 years and 82 days, O'Callaghan became the youngest winner of the 100m freestyle since 1991, when Nicole Haislett of the United States won the title at 18 years and 22 days.

O'Callaghan said she had suffered badly from pre-race nerves.

"The worst ever," she said.

"I was panicking in my bed, having a little bit of a cramp in my leg, just feeling dizzy, feeling out of it, starting to panic, but I knew I had my teammates there... I guess that kind of uplifted me for the race."

King, who has dominated the 50m and 100m breaststroke, grabbed her first gold at 200m, the longest breaststroke distance.

"It's really nice to be able to complete the set, I guess I'm a distance swimmer now," said King.

Murphy ended a long streak of duller coloured medals when he won the men's 200m backstroke.

Since grabbing two individual Olympic golds in Rio in 2016, the American had collected six silvers and two bronzes in major global championships, including a silver in the 100m backstroke in Budapest.

Murphy won in 1:54.52, 0.64sec ahead of Briton Luke Greenbank with another American, Shaine Casas third.

At 26, Murphy is one of the veterans on the American team.

"It's a really cool feeling," he said.

"I think back to 2014 when I was the youngest member in the team, now it's cool to think that I am in a position to mentor the young guys.

"In eight years from now there will be somebody else leading the team, and they will probably look back at some of the things I did to help them out."

Stubblety-Cook, the Olympic champion and world record holder, came from last after the first lap to win the men's 200m breaststroke.

Dutchman Caspar Corbeau started off at world record pace, but faded and Stubblety-Cook, Yu Hanaguruma and Erik Persson, who had conserved energy at the back, came through.

The Australian won in 2:07.07, 1.31sec ahead of the Japanese and the Swede who tied for the silver.

"After 35 metres I thought 'Oh no, have I underdone this?" said Stubblety-Cook. "I knew they would go out really fast so I just wanted to hold on and stick to them."

The pace will quicken on Friday with six finals.

Two will decide who inherits titles that Caeleb Dressel has owned.

In the 100m butterfly, which Dressel won in the last two world championships and the Tokyo Olympics, Hungarian star Kristof Milak, who dominates the 200m fly, qualified comfortably the fastest. He seems set to provoke delirium in what he calls 'my pool' as he upgrades from his habitual silver in the shorter distance.

In the 50m freestyle, the picture is cloudier. Brazilian Bruno Fratus, second to Dressel in the last two world championships, tied for eighth in Thursday's semis and was then eliminated in a swim-off by Frenchman Maxime Grousset.

Briton Benjamin Proud qualified fastest.

Katie Ledecky is poised to add to her haul of 21 world championship medals, a women's record, in the 800m freestyle, an event she dominates.