Serena, who will celebrate her 36th birthday on Sept. 26, entered St. Mary's Hospital in West Palm Beach, where a full-length class was booked, according to CBS's local channel.
Serena, who has won 23 major titles since the start of the professional system, was first seen on Thursday night, the channel said.
In December 2016, Serena announced her engagement to Alexis OHanian, co-founder of the social networking site Reddit, and then stopped playing after about a month of crowning the Australian Open, the first Grand Slam.
The American actress revealed in April that she was pregnant and posted on social media photos with a puffy belly and indicated that her birth coincided with the opening of Flushing Meadows, the last Grand Slam tournament that kicked off on Monday.
Serena promised in August to return to the Australian Open to defend her title in January 2018, just three months after her first child was born, and a year after her 23rd title title, she was just one title behind Australian qualifier Margaret Curt.