The tennis star is said to have needed essential treatment for a haematoma, which is when blood collects outside of a blood vessel. The haematoma is thought to have been caused by complications following the discovery of a blood clot - or pulmonary embolism - in Serena's lungs last week.
Thankfully Serena is reported to be recovering well, and her condition is in no way thought to be life threatening. The pulmonary embolism is thought to be linked to problems with Serena's foot, after she stepped on glass at a Milan restaurant. Since then, she has had surgery at least twice to correct the problem.
So what exactly is a pulmonary embolism? A blood clot that affects the arteries in the lungs, most pulmonary embolisms are thought to be caused by clots that originate from the deep veins in the legs (in other words, a deep vein thrombosis, or DVT).
The condition is thankfully rare. But if left untreated it can cause serious damage in the lungs and - in some cases - can be fatal (according to the Office for National Statistics pulmonary embolism causes 0.6% of all deaths).
Being inactive for long periods of time - such as during a long-haul flight - can increase the risk of clotting (Serena's clot was reportedly discovered after she arrived at Los Angeles from New York, which is a six-hour trip). Having surgery could also increase your risk, especially if you're not able to move around much afterwards.
To reduce your risk of developing a pulmonary embolism, experts recommend not smoking, eating a healthy, low-fat diet, getting regular exercise and maintaining a healthy weight.