Spokeswoman Nicole Chabot tells the magazine that the blood clot in Williams' lungs was discovered last week, and that she had "emergency treatment (Monday) for a hematoma she suffered as a result" of that treatment.
Williams is being treated at a Los Angeles hospital.
"Doctors are continuing to monitor her situation closely to avoid additional complications," Chabot tells the magazine.
Williams has been sidelined by two operations on her right foot after getting cut by glass at a restaurant, and has not played a competitive match since winning Wimbledon on July 3.
