A spokeswoman for Williams, 29, said she was treated for a hematoma after she suffered a pulmonary embolism last week.
The tennis star returned home from the hospital Wednesday afternoon, but doctors continued to monitor her situation to avoid further complications.
The former No. 1 women's tennis player has not played competitively since winning last year's Wimbledon title. She cut her right foot on broken glass at a restaurant shortly after the victory, and her comeback has been repeatedly delayed by complications with the injury since.
Williams had surgery after initially hurting her foot and pulled out of the U.S. Open. She resumed practicing in September, but kept pushing back her return and needed an additional operation in October.
Williams missed the Australian Open in January, where she was the two-time defending champion.
In a statement released Wednesday, Williams described her health scares as "extremely hard, scary, and disappointing."
The 13-time Grand Slam champion said in a statement: "I know I will be OK, but am praying and hoping this will all be behind me soon. While I can't make any promises now on my return, I hope to be back by early summer. That said, my main goal is to make sure I get there safely."
The Associated Press contributed to this report.