Venus Williams, 31, has spoken out about the autoimmune disease that caused her to drop out of the U.S. Open yesterday minutes before she was due to go on court.
Miss Williams was diagnosed with Sjogren's syndrome two weeks ago - after years of being misdiagnosed with other conditions.
'For years I felt that I didn't have enough stamina and then, four years ago, I felt like I was not getting enough air but I was diagnosed with exercise-induced asthma,' she said.
'The medicine for asthma never worked.'
Eventually, as her symptoms progressed doctors got to the bottom of the problem.
The American athlete now suffers from fatigue, swollen joints, dry eyes, dry mouth and heavy limbs.
In an exclusive interview with Good Morning America, Miss Williams said, despite the fact there is no cure for Sjogren's, she feels positive about the future and returning to tennis.
'My normal might not be everyone else's normal but I feel like now I can get better,' she said.
'It makes me want to get up and fight harder every day,' added Miss Williams whose sister Serena has battled clots in her lungs.
Sjogren's syndrome is difficult to diagnose - it occurs when white blood cells attack healthy organs instead of diseased areas of the body.
The 31-year-old American has won seven Grand Slam titles, including at Flushing Meadows in 2000 and 2001.
'I enjoyed playing my first match here, and wish I could continue but right now I am unable to,' Miss Williams said in a statement released by the tournament.
'I am thankful I finally have a diagnosis and am now focused on getting better and returning to the court soon.'
She was supposed to face 22nd-seeded Sabine Lisicki in the second round on Wednesday.
Miss Williams cited a virus when withdrawing from hard-court tuneup tournaments since losing in the fourth round at Wimbledon in late June.
She returned to action on Monday, beating Vesna Dolonts 6-4, 6-3 in the first round in New York, then said: '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.'
According to the Sjogren's Syndrome Foundation, the disease is a chronic autoimmune illness in which people's white blood cells attack their moisture-producing glands.
In rare cases, it can cause arthritis and joint pain, said Dr John Fitzgerald, director of clinical rheumatology at UCLA.
Dr Fitzgerald is not involved in treating William, but said 'it does not seem life-threatening or career-ending'.
Williams arrived at the Billie Jean King National Tennis Center on Wednesday hours before her match was scheduled to begin and tried warming up by hitting balls.
When Miss Williams left the site shortly before 5 pm, she was asked by reporters whether she would say anything.
She smiled and waved and shook her head to indicate, 'No', then climbed into the back of a tournament transportation car and rode away.
'All of us came with the full expectation she'd be playing today. She was geared up to play her match,' said Miss Williams' agent, Carlos Fleming.
'I just hope she's OK,' Mr Fleming added, 'and I hope she's healthy and going to be fine'.
Despite all of her past success, including a brief stint at No. 1 in the rankings, Miss Williams was unseeded at the U.S. Open, because she has fallen to 36th after a year of little action.
Since reaching the semifinals at last year's U.S. Open, she has played only 11 matches, and the WTA projects that her ranking now will slide out of the top 100.
Her younger sister, Serena, a 13-time Grand Slam champion, is scheduled to play her second-round match today.
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