The reigning Wimbledon champion had said after her surgery for a blood clot that travelled from a leg into her lungs that she was hopeful of a summer return, but was unsure exactly when she might be back on the court.
The former world number one, a 13-time Grand Slam champion, would need to be ready by June 20 to defend her Wimbledon title. The women's final is set for July 2.
Williams has not played a top-level match since last year's Wimbledon final after cutting her right foot on broken glass at a restaurant last July.
