The pair led the way in the semi-finals, but it was American Torri Huske who led going into the closing stages.

However, O'Callaghan of Australia finished strongly, and took her first individual World Championships gold at the age of 18 in 52.85sec.

Sjöström of Australia was 0.17 further back, although her second-placed finish means that only American legend Michael Phelps has won more individual medals at the World Championships.

Huske came through in third in 53.04.

Olympic champion and world record holder Zac Stubblety-Cook took Australia's gold medal haul at the World Championships up to four with victory in the men's 200m breaststroke.

A characteristically strong final split of 31.98 enabled Stubblety-Cook to win the final by more than one second, and the race featured shared medallists in Budapest for the first time as Japan's Yu Hanaguruma and Sweden's Erik Persson both claimed silver.

Stubblety-Cook clocked 2min 7.07sec, with his closest challengers finishing in 2:08.38.

The other three swimming golds on the sixth day of competition went to the United States, taking their total at the Championships up to 14.

Most dramatically, Lilly King earned her ninth gold at the World Championships and first in the women's 200m breaststroke.

With 50m remaining, King was fifth, but a 36.36 final split propelled her to victory in 2:22.41.

Jenna Strauch of Australia took silver in 2:23.04, while the US' Kate Douglass was third in 2:23.30.

Ryan Murphy of the US triumphed in the men's 200m backstroke, having taken silver at the previous two World Championships.

The man who beat him on both of those occasions and at the Tokyo 2020 Olympics Evgeny Rylov was not in Budapest to defend his title, with Russia banned from the event because of the invasion of Ukraine and the double Olympic champion suspended by FINA for nine months for attending a pro-war rally in March.

Murphy's time of 1:54.52 staved off a late challenge from Britain's Luke Greenbank, the Olympic bronze medallist taking silver in 1:55.16.

Shaine Casas of the US completed the podium in 1:55.35.

The US also continued their dominance of the relays at the World Championships, with their success in the men's 4x200m freestyle relay their fourth from a possible five.

Drew Kibler, Carson Foster, Trenton Julian and Kieran Smith dominated the race and claimed gold in more than three seconds with their time of 7:00.24.

Defending champions Australia placed second in 7:03.50, with Olympic champions Britain finishing third in 7;04.00.

Earlier in the day at the Széchy Outdoor Pool, China's Wang Liuyi and Wang Qianyi won their third artistic swimming gold medals of the tournament, following up success in the women's duet technical and team technical routine with a victory in the duet free.

Their routine earned a score of 95.5667, with nearest rivals Maryna Aleksiiva and Vladyslava Aleksiiva of Ukraine notching 94.5667.

As they did in the duet technical, Austria's Anna-Maria Alexandri and Eirini Alexandri completed the top three in 92.8000.

Tomorrow is set to mark the penultimate day of swimming and artistic swimming finals in Budapest.

The women's 50m butterfly and men's 50m freestyle finals are scheduled to start tomorrow's evening session in the swimming, followed by the men's 100m butterfly and women's 200m backstroke, and then the women's 800m freestyle and mixed 4x100m freestyle relay.

The women's team free competition is scheduled in artistic swimming.