Australian backstroke star Kaylee McKeown pulled out of the Commonwealth Games and Pan Pacific Swimming Championships on Friday, citing fatigue and loss of breath.
The reigning Olympic, world and Commonwealth 100m and 200m backstroke champion said she was devastated to miss the events where she was red-hot favourite.
"I am gutted to medically withdraw," said the 24-year-old, widely considered the greatest backstroker of all time.
"What I thought a few months ago was the flu has turned out to be my body fighting glandular fever.
"I was sick going into trials and I just have not got better and I am worried about pushing myself so hard that I end up with chronic fatigue.
"It was a hard decision to make but it was the right one."
Swimming Australia head coach Rohan Taylor said McKeown's health must be the priority.
"We all know how fierce and competitive she is -– and she showed that by performing the way she did at trials," he said.
"Kaylee is a once in a generation athlete. She is what we call a racer.
"We are going to miss her on team not just in the pool but out of the pool with her experience and leadership but I personally feel, respect and support her decision to prioritise health."
Australia will now take a 59-strong swim team to the Commonwealth Games in Glasgow from July 23-August 2 and 43 to the PanPacs in Irvine, California, from August 12-15.