Her agents confirmed Wednesday that Williams was diagnosed with a pulmonary embolism last week and later needed treatment for a hematoma. The 13-time Grand Slam champion hasn't played an official match since winning Wimbledon last July because of a foot injury she sustained not on the court but at a restaurant.
Her latest health problems have been "extremely hard, scary and disappointing," Williams said in a statement. "I am doing better. I'm at home now and working with my doctors to keep everything under control. 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."
People magazine first reported on Williams' condition, quoting spokeswoman Nicole Chabot as saying Williams underwent "emergency treatment" Monday for a hematoma suffered as a result of treatment for "a more critical situation," the pulmonary embolism.
The 29-year-old Williams was treated at a Los Angeles hospital then returned to her home in the city.
"Thankfully everything was caught in time," her agents said in a statement. "With continued doctor visits to monitor her situation, she is recuperating at home under strict medical supervision."
Williams' mother, Oracene Price, tweeted: "Thank you for your concern. She is fine."
The tennis star attended Sunday night's Elton John AIDS Foundation Academy Awards Viewing Party. On Tuesday night, Williams posted on her Twitter account, "Tough day." A few minutes later, she retweeted Kim Kardashian.
The younger sister of seven-time major champion Venus Williams has been out of competition since she cut her right foot on broken glass at a restaurant shortly after winning her fourth Wimbledon title July 3. Her comeback has been repeatedly delayed by complications with the injury.
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.
Chabot told the magazine the embolism was discovered after Williams returned to Los Angeles from New York "for doctor appointments for the ongoing issues with her foot."
Dr. Mark Adelman, chief of vascular surgery at NYU Langone Medical Center, said a patient with a pulmonary embolism would need to take an anticoagulant for six to 12 months but could play sports on the medication.
"A blood clot can occur in any vein or extremity, most commonly in the leg, and can travel to the lung," Dr. Adelman wrote in an e-mail. "Prior surgery, air travel, prolonged sitting, birth control pills, obesity and pregnancy can predispose a patient to a blood clot in the leg that can travel to the lung."
Adelman said if a clot-dissolving agent is used to treat an embolism, it can result in bleeding around the catheter used to deliver the drug. Williams' agents said the hematoma was removed. Williams has a wide range of business, fashion and charitable interests that keep her in the public eye even when she's not on the court. Since winning her first Grand Slam title in 1999, she has struggled with injuries on several occasions only to come back to win more championships.
AP Tennis Writer Howard Fendrich contributed to this report.
According to the U.S. National Library of Medicine, a pulmonary embolism is a sudden blockage in a lung artery. The cause is usually a blood clot in the leg called a deep vein thrombosis that breaks loose and travels through the bloodstream to the lung.