NEW YORK  — 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 causes 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.”