The 13-time Grand Slam singles champion and former No. 1-ranked player has not played an official match since she won the title at Wimbledon in July.
On Tuesday, Williams tweeted: "Cool news guys stay tuned.. Ill update u with a pic.. U ready??''
Then, a little later, she tweeted again: "Look who I spotted on the court. Her first day back...'' That posting came with a link to a photo of Williams in a neon pink bodysuit, standing at the baseline and midway through her service motion with a racket in her right hand.
Her agent, Jill Smoller, confirmed Williams put in some work Tuesday.
"She was out hitting some balls today. Taking it day by day depending on how she's feeling and was very happy to be back on the court,'' Smoller wrote in an email to The Associated Press.
Shortly after claiming her fourth singles championship at the All England Club last year, Williams cut her foot on glass at a restaurant, an injury that led to two operations. The second surgery was in October, and she said she spent 10 weeks in a cast and 10 weeks in a walking boot.
The 29-year-old American was diagnosed in February with blood clots in her lung. After that, she said she needed treatment for a hematoma - a gathering of blood under the skin - on her stomach.
Williams has not said when she might return to competition.
Despite all of the time away from the tour, she is still at No. 10 in this week's WTA rankings.
Her older sister Venus is 15th this week; she hasn't played since January because of a hip injury.