Venus Williams of the United States returns the ball against Vesna Dolonts of Russia during Day One of the 2011 US Open at the USTA Billie Jean King National Tennis Center on August 29, 2011 in the Flushing neighborhood of the Queens borough of New York City.

UPDATE: Venus Williams has released a statement explaining her sudden withdrawal from the U.S. Open tennis tournament in New York. She says she’s recently been diagnosed with Sjogrens Syndrome, an autoimmune disease that affects energy level and causes joint pain. She expressed disappointment about having to pull out of the event, but says she’s relieved to now have an explanation of her recent symptoms.

Earlier story…

*After winning her first round match at the U.S. Open, Venus Williams has pulled out of the tournament due to an unspecified illness.

Venus, who came to the U.S. Open unseeded, ranked just 36th in the world after spending her summer withdrawing from hard-court tournaments because of a viral illness.

On Monday, Williams started slowly before beating Vesna Dolonts, 6-4, 6-3.

Serena Williams of the United States reacts after a play against Bojana Jovanovski of Serbia during Day Two of the 2011 US Open

Meanwhile, Serena Williams cruised to her first round win against Serbian Bojana Jovanovski, posting a 6-1, 6-1 victory in less than an hour on the final match of Tuesday night.

Serena is seeking her fourth U.S. Open title after missing last year’s event while recovering from foot surgery.