French world number 31 Alize Cornet was given a code violation for taking her shirt off on court during her US Open first-round defeat by Johanna Larsson.
Cornet changed her shirt during the break between the second and third sets, but realised it was back-to-front and quickly switched it on court.
But the chair umpire gave her a warning - despite male players being permitted to change their shirts on court.
Cornet lost 6-4 3-6 2-6 to Sweden's Larsson in the searing New York heat.
On Tuesday, temperatures close to 38C (100F) at Flushing Meadows were made more stifling by humidity levels of over 50%, with five male players forced to retire due to heat-related issues.