American performances at the swimming world championships have been eclipsed by an Australian gold rush but backstroker Justin Ress said Saturday there was no panic in the US camp.

The Americans have claimed only three gold medals in six days of competition so far in Fukuoka, compared with 10 for Australia and five for China.

Ress is aiming to defend his men's 50m backstroke title after posting the fastest time in the morning's heats in 24.18sec, ahead of Greece's Apostolos Christou on 24.48 and American world record-holder Hunter Armstrong on 24.49.

Ress said the Americans were "swimming really well as a team" despite the lacklustre gold medal count.

"You can't control what other teams are doing," he said.

"Australia is lighting it up, it is what it is.

"We're just doing the best we can and that's all you can do -- have fun."

American Katie Ledecky won the women's 1,500m freestyle, and is the red-hot favourite going into the 800m freestyle final later on Saturday.

Kate Douglass took gold in the women's 200m individual medley, while Ryan Murphy won the men's 100m backstroke.

The Americans have also claimed 13 silver medals and nine bronzes so far in Japan.

Ress said he was not worried about what his teammates do and was "just focused on myself".

"That's what I've been doing for the past year and it's been pretty good, so I'm just going to keep doing that," he said.

Elsewhere in the morning heats, Italian world record-holder Benedetta Pilato swam the fastest time in the women's 50m breaststroke.

Pilato clocked 29.60sec to finish ahead of compatriot Anita Bottazzo on 30.02 and American Lilly King on 30.05.

Sweden's Sarah Sjoestroem was quickest in the women's 50m freestyle.

Sjoestroem, the world record-holder and defending champion, touched the wall in a time of 23.93sec.

She was followed by Australia's Shayna Jack on 24.02 and American Abbey Weitzeil on 24.29.

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