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Tennis star Venus Williams said today she’s  pulling out of the U.S. Open due to a disease that “causes fatigue and joint pain.”

“I have been recently diagnosed with Sjogren’s Syndrome, an autoimmune disease which is an ongoing medical condition that affects my energy level and causes fatigue and joint pain,” the two-time champion said in a statement.

Sjögren’s syndrome is a chronic condition in which an individual’s disease-fighting cells attacks the moisture-producing glands in the body.

Today, as many as 4 million Americans are living with this autoimmune disease, according to the Sjögren’s Syndrome Foundation, and the disease is far more prevalent in women.

Typical symptoms of the condition are dry eyes and dry mouth, but Sjögren’s has also been implicated in central nervous system issues, extreme fatigue, joint pain and problems with various organs.

Williams was set to play against Sabine Lisicki, today.

On Monday, Williams beat Vesna Dolonts 6-4, 6-3.

“I enjoyed playing my first match here and wish I could continue but right now I am unable to,” Williams said in the statement.

Earlier this summer, Williams, 31, also had to withdraw from two tournaments due to a virus, the Associated Press reported.

Williams said she was disappointed she had to pull out of the tournament but added she was thankful to finally have a diagnosis and is “now focused on getting better and returning to the court soon.”

ABC News’ Dan Childs contributed to this report.