Spokeswoman Nicole Chabot told the magazine that Williams "underwent emergency treatment for a hematoma suffered as a result of treatment for a more critical situation."
Williams suffered from a pulmonary embolism last week, Chabot said. The 29-year-old Williams is being treated at a Los Angeles hospital.
"Doctors are continuing to monitor her situation closely to avoid additional complications," Chabot told the magazine.
The winner of 13 Grand Slam titles, Williams 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 hasn't played an official match since winning Wimbledon for the fourth time July 3. 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.
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."
Second-ranked Kim Clijsters tweeted Wednesday: "Just read about Serena!!!! Very scary, hope she is ok!fingers crossed!"
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.