The sports star was forced to seek emergency treatment at Cedars-Sinai Medical Center in Los Angeles for a haematoma - a pooling of blood caused by a haemorrhage.
It followed a health scare last week (ends27Feb11) when Williams was diagnosed with a blood clot in her lungs following a trip to New York for treatment to her ongoing foot injury.
Williams' representative tells People.com, "(On) Monday Serena Williams underwent emergency treatment at Cedars for a haematoma 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."
The 29 year old has been forced to take time away from tennis after undergoing surgery for a foot injury in July (10).