The tennis star and Aneres fashion designer was found to have a blood clot in the lungs, which is potentially fatal.
Her rep Nicole Chabot told People magazine: "Monday Serena Williams underwent emergency treatment at Cedars for a hematoma she suffered as a result of treatment for a more critical situation.
"Last week, Serena suffered from a pulmonary embolism which was discovered upon her return to L.A. She had been in New York for doctor appointments for the ongoing issues with her foot. Doctors are continuing to monitor her situation closely to avoid additional complications."
"Tough day...." she Tweeted late Tuesday night, PST.
Williams, 29, was last seen at Elton John's Oscars viewing party in Los Angeles on Sunday night. She is known for her expressive personal style and fashion industry collaborations, including a new line of nail polish.