The apparel giant tweeted an image of Williams, who was wearing the outfit on the tennis court, with a message of support for the 23-time Grand Slam champion:
Bernard Giudicelli, French Tennis Federation president, told Tennis Magazine (h/t the Associated Press) the dress code was being introduced because "sometimes we've gone too far."
He added that "one must respect the game and the place."
Williams responded to the decision Saturday, via ESPN:
After defeating Kristyna Pliskova in the first round at Roland Garros, Williams told reporters there was a medical reason behind her decision to wear the outfit.
"I had a lot of problems with my blood clots and, God, I don't know how many I have had in the past 12 months," she said. "So it is definitely a little functionality to it. I have been wearing pants in general a lot when I play so I can keep the blood circulation going."
Williams had numerous health problems, including a pulmonary embolism, after giving birth to her daughter last September.
This year's French Open was Williams' first appearance in a Grand Slam tournament after announcing her pregnancy in April 2017.