The tennis great had kept the news that she is expecting a secret until Wednesday, when she posted a photo on Snapchat of her growing baby bump with the caption “20 weeks."
Her rep confirmed the news: "Serena is expecting a baby in the fall."
In December, the 35-year-old announced her engagement to Reddit co-founder Alexis Ohanian, who is the father of the child. It is the first child for the couple and the gender has not yet been revealed.
Williams will miss the rest of the 2017 tennis season, but she's expected back on the court next year to compete for the dual role of greatest tennis player in the world and a mom.
On Thursday morning, Williams took to Instagram to post a famous Spider-Man quote: “With great power comes great responsibility,” along with a photo of her looking into the ocean from a bluff in Mexico with her hand on her back.