Venus Williams of USA defeated Vera Dolonts of Russia 6-4, 6-3 in women's singles 1st Round at the 2011 US Open. [Source:Sina.com]

Venus Williams quit the U.S. Open on Wednesday after revealing she was suffering from a chronic illness that threatens her future in the sport.

Two-time US Open champion Venus Williams announced her withdrawal from this year's tournament just before she was due to play her second round match against No. 22 Sabine Lisicki of Germany on Wednesday, August 31.

Williams had missed much of the 2011 tennis season with a hip and abdominal injury and then was hit with a virus late in the summer. In a statement regarding her withdrawal, Williams said she has finally received a diagnosis for her illness.

"I have recently been diagnosed with Sjogren's Syndrome, an autoimmune disease which is an ongoing medical condition that affects my energy level and causes fatigue and joint pain," she said in a statement.

Robin Soderling also pulled out on Wednesday, because of a mystery virus that has plagued him for weeks, becoming the biggest casualty so far in the men's draw.

The Swede was seeded sixth and regarded as one of the few players able to challenge the big four of Roger Federer, Rafa Nadal, Novak Djokovic and Andy Murray.