Venus Williams of the US reacts to a point during her first round match against Vesna Dolonts of Russia at the US Open tennis tournament in New York, Aug 29, 2011. [Photo/Agencies]

NEW YORK - Venus Williams expects to return to tennis court despite being diagnosed with Sjogren's Syndrome that causes fatigue and led to her decision to withdraw from the US Open, but she is sure to face a tough fight to be back to the highest levels.

The 31-year-old American, who was diagnosed with a strength-sapping  autoimmune disease that left her barely able to lift her arms, pulled out  of the US Open on Wednesday, said in a statement issued at the Open in New York and reiterated on Thursday on ABC's "Good Morning America" that she planned to return to professional tennis.

But the seven-time Grand Slam singles champion is likely to run into  dead end in her efforts of reaching top again. The Sjogren's syndrome, which is mainly found in women, can cause extreme fatigue and joint pain  and affect internal organs, according to Arthritis Research U.K.

Williams Simon Bowman, who runs a clinic for people who have the disease and is medical president of the British Sjogren's Syndrome Association, said that she faces a tough task.

"If you've got a desk job, you might be able to cope with any of these things," said Bowman, a rheumatology consultant at England's University Hospital Birmingham.

"But it might be quite difficult to deal with if you're a top-flight athlete."

Williams, who won the US Open in 2000 and 2001, withdrew shortly before her second-round match against Sabine Lisicki of Germany.

"I had a tough practice, and I was sitting there and it was an effort to just lift my arms," Williams said on the ABC morning show.

She said she was diagnosed a few weeks ago after years of struggling  with her stamina. Some of the long-term treatments could take "three to six months" to start working, Williams said.

What helps at the moment is Williams feels getting better, which  contributes to her confidence build-up for a full return.

"The good news for me is that now I know what's happening after spending years not knowing," Williams said. "Now that I know, I have the chance to get better."

Sjogren's syndrome can lead to inflammation in the muscles and lungs,  make patients feel lethargic and cause dryness in the mouth and eyes, Bowman said.

Although Sjogren's isn't life-threatening, it is an incurable condition where the immune system starts attacking glands that produce tears and saliva instead of fighting infection, according to Arthritis Research U.K.

Around a half-million people in the U.K. suffer from the condition, which is usually diagnosed in women between the ages of 40 and 60. The American College of Rheumatology's website said between 400,000 and 3.1 million people are affected in the United States.

Patients with the condition "feel tired all the time, they feel washed out," Stephen Porter, a professor of oral medicine at UCL Eastman Dental Institute in London and a former council member of the British Sjogren's Syndrome Association, said in an interview.

Playing professional tennis, which requires practice of up to five or six hours a day, "might be tricky," Porter said.

Williams is relatively young to have been diagnosed with the disease, according to Bowman.

"It's a very variable disease, and there is typically inflammation in  the glands that produce tears and saliva," Bowman said.

"But in younger people such as Venus Williams it can have more involvement of other parts of the body. It can lead to general fatigue, which is of course a major problem if you are a high-level sports person, it can cause problems with skin rashes, the breathing, the nervous system, the muscles, it can cause arthritis."

"The dryness I don't think will affect her ability to play tennis," said Porter. "However, if her mouth is dry, it can affect her sleep patterns and that might interfere in the activity of a sports person. That might be a bit of an issue."

Although the dryness symptoms can be relieved with eye drops and moisturizing gels placed in the mouth, there currently isn't a treatment  for fatigue, Porter said.

Williams won Wimbledon five times and formerly was ranked No. 1 on the WTA tour. Now ranked 36th, she struggled with injuries all season and has played only four tournaments. She was unseeded at the US Open for the first time since 1997.