Her sister Venus Williams also confirmed the birth during an interview before her match at the US Open on Friday when asked about Serena having a baby girl and her feelings on becoming an aunt. She said, "Obviously I'm super excited, words can't describe."
The US Open congratulated Williams on its Twitter page, "@serenawilliams won a new title on Day 5 at the #USOpen: Mom."
Serena's coach Patrick Mouratoglou announced on his verified Twitter account, "Congratulations @SerenaWilliams for your baby girl. I am so happy for you and I feel your emotion. Recover well & enjoy without limitation."
He also wished her a speedy recovery. "We have a lot of work ahead of us."
Serena or fianc Alexis Ohanian, co-founder of Reddit, have not issued an official statement on social media.
The couple received congratulations from Beyonce on Instagram and US Open Tennis on Twitter.