Katinka Hosszu had a packed schedule lined up at Rio 2016 and has already won two golds, but she did not start her women's 200 metre butterfly heat on Tuesday.
Hosszu set a new world record when claiming gold in the 400m individual medley on Saturday, and she topped the podium after the 100m backstroke final on Monday too.
The Hungarian - nicknamed the Iron Lady - was set to contest five individual medals at the Games but, with an appearance in the 200m individual medley final coming later on Tuesday, she opted to drop the 200m butterfly.
A spokesman for the Hungarian team said on Tuesday: "There is absolutely no problem with Katinka. She just decided that in such a busy schedule, it would be best to rest and be prepared for the race tonight."
Hosszu is expected to take part in the 4x200m freestyle relay, the heats of which take place on Wednesday.
The Hungarian is also scheduled to take part in the 200m backstroke heats on Thursday.