Gwangju - Olympic champion Katie Ledecky has pulled out of Tuesday's world 1 500 metres freestyle final and 200m heats on "medical grounds," Team USA said.
The 22-year-old, who has won 14 world titles, lost her 400m crown to Australian teenager Ariarne Titmus at the weekend in a stunning upset on day one of the pool competition in Gwangju.
Ledecky opted not contest Tuesday morning's 200m freestyle heats and after assessing her condition decided to skip the evening's 1 500m final - an event she has dominated at the past three world championships.
Team USA officials told AFP Ledecky had not withdrawn from the remainder of the championships - her 800m freestyle heat is scheduled for Friday morning.
But they did confirm the 14-time world champion has complained of feeling unwell since arriving in South Korea.
"Katie has not been feeling well since arriving in Gwangju on July 17 and these precautionary measures are being taken to ensure her well-being and proper recovery and to allow her to focus her energy on an abbreviated schedule," said National Team Managing Director Lindsay Mintenko.
Canada's Taylor Ruck, the Commonwealth and Pan Pacific champion, and Australian Emma McKeon, who took silver at the 2017 world championships, also withdrew from the 200m to better manage heavy schedules this week.
Titmus, who posted the third fastest time of the 200m free heats with Sweden's Sarah Sjostrom setting the pace, reacted with disappointment to Ledecky's pull-out.
"Yeah a little bit because you know three girls pulled out - probably my top three challenges," said the Aussie.
"It's a little bit easier now but (Italy's world champion Federica) Pellegrini is still in there so it's still a tough challenge."