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Williams, 35, whose partner is Reddit co-founder Alexis Ohanian, 34, was admitted to the St. Mary's Medical Centre in West Palm Beach on Wednesday, reports the BBC. She gave birth on Friday night.
The 23-time Grand Slam winner said last month she was planning to return to tennis for the Australian Open in January.
Wishes have been pouring in from sports stars and celebrities.
News of the birth came as her sister Venus prepared to go out on court at the US Open.
"Obviously I'm super-excited," Venus said, adding "Words can't describe it."
Pop star Beyonce and Spanish tennis star Rafael Nadal were also among those congratulating Williams on social media.
"Congrats Serena! So much joy for you!" Nadal tweeted.
Serena admitted she had revealed her pregnancy to the world in April by accident, after mistakenly uploading a photograph on Snapchat.
Serena and Ohania was engaged last December at an Italian restaurant where they first met.
The two have yet to announce a wedding date.
IANS