Venus Williams withdrew from the U.S. Open shortly before her second-round match was scheduled to begin Wednesday, suffering from an autoimmune disorder that causes fatigue and joint pain.

The condition, known as Sjogren’s syndrome, is not life-threatening and need not be career-ending for a world-class athlete. But it was unclear in the aftermath of Williams’s announcement whether or when she would return to competition.

Williams, 31, gave no hint that she was contemplating retirement in the statement she issued, saying: “I enjoyed playing my first match here and wish I could continue, but right now I am unable to. I am thankful I finally have a diagnosis and am now focused on getting better and returning to the court soon.”

The news came as a shock at the Billie Jean King National Tennis Center, where Williams, a two-time U.S. Open champion, made an impressive return to competition Monday after pulling out of the hard-court season this summer with what was described as a virus.

Williams showed no sign of illness Monday in cruising past Vesna Dolonts, 6-4, 6-3, to advance to Wednesday’s second round.

Hall of Famer Chris Evert, now an ESPN analyst who provided commentary for that match, said she was “surprised and shocked” by the development.

“She looked healthy to me,” Evert said of Williams’s performance on Monday. “She was playing well. She was moving well. She didn’t look like she was gasping for air, so I was very surprised. I think she would have had a tough match on her hands today [against 22nd seed Sabine Lisicki]. She would have had to have been healthy to win that match.”

Sjogren’s syndrome is a disease in which a person’s white blood cells attack moisture-producing glands. Roughly 4 million Americans suffer from it — 90 percent of them women, according to Steven Taylor, chief executive of the Sjogren’s Syndrome Foundation.

The most common symptoms are dry eyes or dry mouth, but the disease can affect all organs and body systems. It can also trigger moderate to severe fatigue as well as joint pain, Taylor added. While it can’t be cured, the symptoms can be managed with medication.

“Surely the disease does affect quality of life,” Taylor said. “It’s life-altering but not life-threatening. We see people who are very healthy, doing the things they have always done — but just monitoring themselves and taking care of themselves.”

It’s unclear how long Williams has suffered the symptoms or when the condition was diagnosed. Following her straight-sets victory Monday, she mentioned a recently diagnosed health challenge.

“A lot of the battle is just trying to be fit and stay healthy,” Williams said. “Sometimes I’ve been losing that battle a lot. . . . No one is more in one-match-at-a-time mode than me now at this tournament. It will just be one match at a time, for sure.”

Evert said she would be surprised to see Williams return this year given that the U.S. Open marks the symbolic end to the season in many players’ minds.

“It gets harder and harder when you get older to commit because you’re not as hungry,” said Evert, who retired at 34. “Looking ahead to 2012, she’s just going to have to do some soul-searching on how much she wants to play because these [younger] players are getting better and better.”

Also Wednesday, Andy Roddick, the last American to win the U.S. Open’s men’s title (in 2003), survived a scare from little-known compatriot Michael Russell, 33, who has never won any of his seven U.S. Open matches over his 13 years as a pro.

Roddick, who turned 29 Tuesday, is mired in a slump. A shoulder injury in May and an abdominal strain in July sidelined him much of the summer, and his confidence and ranking have suffered badly. He fell from the top 20 for the first time in a decade and is ranked a career-low 21st at the U.S. Open.

After a commanding start Wednesday, Roddick lapsed into an oddly passive stretch, allowing the 5-foot-8 Russell to muscle him around the court while he retreated farther behind the baseline. But with the crowd cheering him on, he prevailed 6-2, 6-4, 4-6, 7-5.

With Roddick’s prospects shaky and Williams’s future in question, a new wave of Americans asserted themselves at the U.S. Open. New Jersey native Christina McHale, 19, scored the biggest victory of her career, knocking off 2007 Wimbledon finalist and No. 8 seed Marion Bartoli, 7-6 (7-2), 6-2.

After her 2-6, 6-3, 7-5 upset of 14th-seeded Dominika Cibulkova, American Irina Falconi pulled out the American flag that she carries in her bag for good luck and waved it in celebration.

“I’ve heard so much about the media talking about American tennis, and I really wanted to portray that there’s a huge wave of American players,” said Falconi, 21, who grew up in New York. “I strongly believe in all that is USA, and I wanted to represent it and show the world that it’s coming. It’s coming! No need to wait any longer.”

Also advancing: 2006 U.S. Open champion Maria Sharapova, who routed Anastasiya Yakimova, 6-1, 6-1, and No. 4 seed Andy Murray, a 7-6 (7-5), 6-2, 6-3 victor over Somdev Devvarman (U.-Va.). Americans John Isnerand 18-year-old Jack Sockalso reached the third round.