Venus Williams has dropped out of the U.S. Open. The 31-year-old two-time champion's abrupt withdrawal on Wednesday came as a surprise to many, who saw Williams as a major contender for this year's title. She's ranked number 36 in the world among female tennis players. Williams explained in a statement that she's suffering from Sjogren's syndrome, an autoimmune disorder that can cause fatigue and joint pain when immune cells attack the glands that produce tears and saliva.Venus Williams was diagnosed with anemia earlier this year, and at the 2007 U.S. Open she complained of fatigue. Before she withdrew, Williams won her first round on Monday, defeating Vesna Dolonts in New York. It was her first match in two months. The Williams family will still have a presence in the tournament- Venus' younger sister Serena Williams is considered a major competitor. Serena's ranked number 27, and has 13 Grand Slam titles to her name.