(Reuters) -Australian juggernaut Mollie O'Callaghan will look to add 100 metres freestyle gold to her 200 crown after finishing fastest in the heats at the world championships in Singapore on Thursday.
O'Callaghan clocked 53.40 seconds at the World Aquatics Championships Arena, with Dutch defending champion Marrit Steenbergen second quickest, 0.13 seconds behind the Australian.
With pregnant Olympic champion Sarah Sjostrom sitting out the event, and Paris runner-up Torri Huske out of sorts, O'Callaghan
is in a good spot to repeat the sprint double she achieved at the 2023 championships in Fukuoka.
American Gretchen Walsh, a medal chance who won 100 butterfly gold on day two, did not take her place at the starting blocks, one of several swimmers scratched from events on day five.
A U.S. spokesperson said Walsh had been pulled from the race as part of "broader race management plans" but said the team would not comment on individual athletes' health.
The U.S. team were hit by a bout of food poisoning on a pre-meet camp in Thailand, and other nations' swimmers have also fallen ill, including Australian Sam Short and Italian Nicolo Martinenghi.
Olympic 100 breaststroke champion Martinenghi, who took silver in the event in Singapore, pulled out of the 200 heats on Thursday.
Russian Aleksandr Zhigalov, competing as a neutral athlete, was top seed into the 200 semi-finals with a time of 2:08.32, edging Japan's Ippei Watanabe by 0.09. China's Qin Haiyang, the 100 winner, qualified fourth.
Canada's Blake Tierney was quickest in the 200 backstroke heats (1:55.17) but Italian Thomas Ceccon, who took silver in the 100, crashed out with the 17th-ranked time.
The U.S. were fastest qualifiers for the women's 4x200 freestyle relay final in 7:49.43, more than two seconds ahead of Australia.
Both swimming powers will be expected to bolster their teams with more experienced campaigners in the final.
China qualified third, with 12-year-old Yu Zidi leading off the team.
Summer McIntosh will be hot favourite to win women's 200 butterfly gold in the evening session, one of five titles on offer on Thursday.
Olympic champion Leon Marchand will swim in the men's 200 individual medley final, a night after smashing Ryan Lochte's long-standing world record with a stunning swim of 1:52.69 in the semis.
(Reporting by Ian Ransom in Melbourne; Editing by Peter Rutherford)
