Venus Williams of the U.S. hits a return to Vesna Dolonts of Russia during their match at the U.S. Open tennis tournament in New York, August 29, 2011.  [Photo/Agencies]

NEW YORK - Venus Williams quit the US Open on Wednesday after revealing she was suffering from a chronic illness that threatens her future in the sport.

The former world number one told officials she was withdrawing from the event less than an hour before she was due to play Germany's Sabine Lisicki in the second round.

Then she dropped the bombshell, with her first public explanation of exactly what has been troubling her.

"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.

Williams said she hoped to resume playing after receiving treatment for the disease but the odds may be stacked against her reaching her best. She is already 31 and has not won a grand slam singles title in more than three years.

I think she's a tough girl," Lisicki said. "I think she'll come back."

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.

All four have sailed through to the second round without any fanfare with Murray launching his campaign with a 7-6 6-2 6-3 victory over Somdev Devvarman of India on Wednesday.

The Scotsman admitted he played below his best but he had little cause to worry as he cruised to victory.

"It's all about just winning," Murray said. "That's all that matters at the end of the day."

Juan Martin Del Potro made his overdue return to the U.S. Open since his stunning victory in 2009. The towering Argentine crushed Italy's Filippo Volandri 6-3 6-1 6-1.

"It's my favourite tournament," Del Potro said. "I'm really happy to get the opportunity to play here again."

Del Potro was unable to defend his title last year after suffering a wrist injury that kept him off the circuit for nearly nine months.

The women's draw, already depleted by the absence of Kim Clijsters and the early defeats of of Petra Kvitova and Li Na, suffered another high-profile loss when eighth seed Marion Bartoli was beaten 7-6 6-2 by American Christina McHale.

Russia's Vera Zvonareva, last year's runner-up and the second seed this time around, was never in any real danger of joining the casualty list even though she had a tough workout against Kateryna Bondarenko of Ukraine before triumphing 7-5 3-6 6-3.