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Williams, 31, said in a statement that she is suffering from Sjögren's Syndrome, an autoimmune disorder that causes fatigue and joint pain.
"I'm really disappointed to have to withdraw from this year's U.S. Open," she said in the statement, adding that her diagnosis is "an ongoing medical condition."
According to the Sjögren's Syndrome Foundation website, the disease affects some 4 million Americans, and nine out of ten patients are women.
The symptoms include dry eyes and dry mouth and it may also cause dysfunction to such organs as kidneys, gastrointestinal system, blood vessels, lungs, liver, pancreas, and the central nervous system. The symptoms occur when white blood cells attack their moisture-producing glands.
Coincidentally, Venus's sister, Serena Williams, dropped out of the U.S. Open last summer, due to injury.
Venus won at Flushing Meadows in 2000 and 2001. In her first match in two months Monday, she defeated Vesna Dolonts 6-4, 6-3 in the first round. She was supposed to face 22nd-seeded Sabine Lisicki Wednesday.
Williams had said she was ill with a virus when she withdrew from tune-up tournaments between Wimbledon and the U.S. Open.
"With proper medical management and treatment, I expect to be in full form once again," Williams said. "I enjoyed playing my first match here and wish I could continue, but right now I am unable to. I am thankful I finally have a diagnosis and am now focused on getting better and returning to the court soon."
