Venus Williams pulls out of U.S. Open due to autoimmune disease

"I'm really disappointed to have to withdraw from this year's U.S. Open," Williams said in a statement. "I have recently been diagnosed with Sjögren's syndrome, an autoimmune disease which is an ongoing medical condition that affects my energy level and causes fatigue and joint pain."

The website for the Sjögren's Syndrome Foundation says as many as 4 million Americans live with the disease. Over 90 percent are women. The disease attacks the body's moisture glands with white blood cells and causes various symptoms ranging from dry mouth to debilitating fatigure.

This recent diagnosis is the latest in a long line of health problems for the 31-year-old tennis star. She's only played four tournaments in 2011 due to an abdominal injury suffered at the Australian Open and a viral illness that prevented her from playing any warm-up tournament prior to the U.S. Open.