Venus Williams withdrew from the US Open before her second-round match against Germany's Sabine Lisicki last night after revealing that she is suffering from an auto-immune disease. The 2000 and 2001 champion, who has been troubled by health problems for the last year, said she had been diagnosed with Sjogren's Syndrome, a medical condition that affects her energy levels and causes fatigue and joint pain.

"I wish I could continue, but right now I am unable to," Williams said in a statement. "I am thankful I finally have a diagnosis and am now focused on getting better and returning to the court soon."

Williams has played in only four tournaments since Wimbledon last year and has dropped to No 36 in the world rankings. She retired with a hip problem in the third round of the Australian Open in January, returned to play at Eastbourne and Wimbledon but then pulled out of two tournaments in the build-up to the US Open because of illness. She won her first-round match against Russia's Vesna Dolonts on Monday but looked well below her best.

Sjogren's Syndrome affects the whole body and can cause problems in the kidneys, lungs, liver, pancreas, blood vessels and central nervous system. Williams did not specify the diagnosis when she was asked earlier this week about the reason she had pulled out of recent tournaments but said: "It was just energy-sucking and I just couldn't play pro tennis. It was disappointing, because I had huge plans for this summer, of course, to improve my ranking. To miss out on all those points was definitely devastating."

Robin Soderling was another high-profile withdrawal from the tournament, pulling out with illness shortly before he was due to play his first-round match yesterday against the Irishman Louk Sorensen. The world No 6 was replaced by a lucky loser, Brazil's Rogerio Dutra Da Silva, who found himself in the second round after Sorensen retired with cramp when trailing in the fourth set.

Sorensen, whose fellow Irishman Conor Niland had to retire from his first-round match against Novak Djokovic because of food poisoning, first felt the problem midway through the first set. "I hit a forehand and suddenly my thumb was cramping, then my legs started to cramp and my left arm, and I had no answer to that," he said. "I don't know if it was the tension."

Marion Bartoli, the world No 9, went out of the tournament when she was beaten 7-6, 6-2, by the 19-year-old American Christina McHale, who beat Caroline Wozniacki in Cincinnati last month.