Volkswacht Bodensee - Australia's Pallister knocked out of women's 800m final by Covid

Pallister, who had qualified second fastest for the 800m freestyle final on Friday, the penultimate evening of competition, was the only one named after the Australian team "put our hand up".

On Thursday, "Pallister felt unwell and tested herself," said a team spokesman.

When the result was positive, she told team doctors, who tested her again, and they told FINA, who performed their own tests and asked the swimmer to isolate, the spokesman said.

A coach on the Australian team also tested positive, but was not one of the eight said the spokesman.

No other team has confirmed a positive test.

The US team said they had tested all athletes before the competition and all unvaccinated team members five days in, and none had tested positive.

Pallister had a slim hope of stopping Katie Ledecky winning a record fifth straight 800m freestyle world title.

In the morning heats, world record holer Sara Sjostrom was fastest in the women's 50m free, an event she won in 2017.

The Swede is chasing a sprint double. She opens Friday evening with a bid for a third straight 50m butterfly gold.

One of the teenage stars of the competition, Italian Benedetta Pilato, was second fastest in women's 50m breaststroke qualifying behind South African Lara van Niekerk.

Pilato, a 17-year-old who won the 100m breaststroke on Monday, is the world record holder in the 50m.

Two-time defending champion Lilly King, who declared "I guess I'm a distance swimmer now," after winning the 200m breaststroke on Thursday, qualified in a tie for ninth.

Defending champion Florian Wellbrock was fastest in the 1,500m semi-finals ahead of the man he has taken in as a training partner in Germany in recent months, Ukrainian Mykhailo Romanchuk.

American Justin Ress topped the men's 50m backstroke qualifying. Italian Thomas Ceccon, who set a world record winning the 100m backstroke on Monday, was third. Another of the 17-year-olds, Pole Ksawery Masiuk, was fifth.

The US, the defending champions and world record holders, were fastest in the mixed 100m freestyle relay semi-finals.

I.Meyer--BTB