Speculation about her pregnancy began earlier Wednesday when Williams posted a picture of herself on Snapchat. She was wearing a one-piece bathing suit with a caption reading "20 weeks." The photo was later deleted.
Williams' spokeswoman, Kelly Bush Novak, ended any doubts in an email to the Associated Press. "I'm happy to confirm Serena is expecting a baby this Fall."
Last month the 35-year-old Williams pulled out of California's BNP Paribas Open due to a knee injury. She last played in late January, winning the Australian Open by beating her sister Venus Williams and capturing her 23rd Grand Slam title. She was likely two months pregnant at the time.
Williams is engaged to Reddit co-founder Alexis Ohanian.
Her sponsorship deals rake in about $20 million and business analysts say her impending motherhood would likely enhance her popularity among corporate sponsors.