The three-times French Open champion was not granted a seed by tournament organisers, triggering criticism from women's groups and tennis pundits.
"This is ridiculous. @SerenaWilliams is a formidable athlete (best ever!) and loving new mother," Trump, daughter of U.S. President Donald Trump, wrote on Twitter.
"No person should ever be penalized professionally for having a child! The #WTA should change this rule immediately. #FrenchOpen."
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
Williams returned to the professional Women's Tennis Association (WTA) tour in March at Indian Wells in Southern California.
She won her first two matches before losing to sister Venus Williams. Two weeks later, she fell to Japan's Naomi Osaka in the opening round of the Miami Open.
(Reporting by Steve Keating in Indianapolis. Editing by Ian Ransom)
