South Africa's Chad le Clos fell sick before his race on the opening day of the second leg of the FINA Swimming World Cup in Doha on Thursday.
Le Clos bagged three medals in the opening leg of the short-course tournament in Kazan this past weekend.
The reigning World Cup men's title holder took to Twitter to share the news of his sudden illness as he failed to start in the men's 100m butterfly.
" I got a fever and vomiting all night, so wasn't able to swim today, gutted not to able to compete but hopefully feel better tomorrow, good luck to rest of athletes ," he said.
Meanwhile, Le Clos' compatriot Ryan Coetzee , who won bronze for SA at this year's Commonwealth Games, finished runner-up in the 100m fly.
Coetzee bagged silver, touching the wall in a time of 52.20 seconds, as American Michael Andrew took gold (51.83) and Mathys Goosen of the Netherlands finished third (52.99).
In the women's 200m butterfly, Carina Brand took bronze with a time of 2:35.58 - behind the powerful Hungarian duo of Zsuzsanna Jakabos (2:10.34) and gold medallist Katinka Hossou (2:09.26).
The second leg of the Swimming World Cup continues on Friday and Saturday.