Williams and husband Alexis Ohanian, co-founder of Reddit, welcomed their daughter Friday afternoon at a hospital in Florida Friday afternoon.
This is Williams' first child.
Williams accidentally announced her pregnancy earlier this year when she accidentally posted a public Snapchat with the caption "20 weeks." She later confirmed her pregnancy in an Instagram post that contained a letter to her daughter.
The United States Tennis Association congratulated Serena in a tweet.
Congratulations to @serenawilliams for giving birth to a baby girl!
— US Open Tennis (@usopen) September 1, 2017
Pop star Beyonce also congratulated Williams with an Instagram post.
More on this as it develops.