Katie Ledecky got back to work with the fastest time in the world championships 1,500m freestyle heats on Monday, the morning after coming second to Ariarne Titmus in an epic showdown.

Ledecky, of the United States, could not match Titmus as the Australian won gold in the 400m freestyle and reclaimed her world record on Sunday night in Fukuoka, leaving her celebrated rivals trailing in her wake.

Ledecky is aiming to add to her record collection of 19 world titles in Japan, and she was straight back in action in the 1,500 heats, coming home in a time of 15min, 41.22sec.

"I just wanted to have a good swim and take care of business this morning," said Ledecky, whose name stands beside the 15 fastest 1,500 times ever.

The women's 400 freestyle was one of the most hotly anticipated races of the world championships on the competition's opening night.

It pitted Olympic champion Titmus against Canada's world record-holder Summer McIntosh and defending world champion Ledecky.

It was the first time the three had gone head-to-head since Titmus won at the Tokyo Olympics two years ago.

Titmus led from start to finish and came home in 3min, 55.38sec to win the title and take back the world record that McIntosh had wrested from her in March this year.

Ledecky said she was "pretty happy with my swim so I'm just keeping the positive momentum going into the rest of the week".

"I was just excited to get the 1,500 started today," she said.

She also said that she was "not concerned at all" that the United States had failed to win any golds on the competition's opening day.

"I think our best days are ahead of us," she said.

"We had a really good morning today, we have a great final session on tap tonight so I think there's going to be a lot of gold, silver and bronze to come."

- Popovici gets started -

Italy's Simona Quadarella swam second-fastest in the 1,500 heats with a time of 15:55.05, while Australia's Lani Pallister was third on 15:58.11.

Romanian star David Popovici got his campaign underway in the men's 200m freestyle.

Popovici, the defending champion in both the 100 and 200 freestyle, swam the third-fastest time in the morning heats, coming home in 1min, 45.86sec.

American Luke Hobson had the fastest time on 1:45.69, followed by Britain's Matthew Richards on 1:45.82.

American Regan Smith and Australia's Kaylee McKeown limbered up for what promises to be an epic battle in the women's 100m backstroke by qualifying first and second from the heats.

Smith, the reigning champion, was quickest in a time of 58.47sec, ahead of McKeown on 58.90 and American Katharine Berkoff on 59.04.

In the women's 100m breaststroke, Lithuania's Ruta Meilutyte was the fastest qualifier with a time of 1min, 04.67sec.

Meilutyte, the 2012 Olympic champion, was followed by Ireland's Mona McSharry on 1:05.55 and South Africa's Tatjana Schoenmaker on 1:05.56.

In the men's 100m backstroke, China's Xu Jiayu qualified with the fastest time in 52.87sec.

Xu, the world champion in 2017 and 2019, finished ahead of Hungary's Hubert Kos on 53.12 and Poland's Ksawery Masiuk on 53.15.

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