Venus Williams pulled out of the U.S. Open shortly before her second-round match Wednesday, saying she was diagnosed with Sjogren’s syndrome, an autoimmune disease that can cause fatigue and joint pain.

The 31-year-old American, the champion at Flushing Meadows in 2000 and 2001, played her first match in two months Monday, when she beat Vesna Dolonts 6-4, 6-3 in the first round. Williams was supposed to face 22nd-seeded Sabine Lisicki on Wednesday.

Williams had cited a virus when withdrawing from hard-court tuneup tournaments between Wimbledon and the U.S. Open.

“I enjoyed playing my first match here, and wish I could continue but right now I am unable to,” Williams said in a statement released by the tournament. “I am thankful I finally have a diagnosis and am now focused on getting better and returning to the court soon.”

According to the Sjogren’s Syndrome Foundation website, the disease is a chronic autoimmune illness in which people’s white blood cells attack their moisture-producing glands. Common symptoms include dry eyes and dry mouth. As many as 4 million Americans have the disease.

In rare cases, it can cause arthritis and joint pain, said Dr. John Fitzgerald, director of clinical rheumatology at UCLA. Fitzgerald is not involved in treating Williams and does not know her symptoms or medical history. But, he said, if Williams has the typical symptoms, “it does not seem life-threatening or career-ending.”

Williams arrived at the Billie Jean King National Tennis Center on Wednesday hours before her match was scheduled to begin and tried warming up by hitting balls.