Former US Open champion Venus Williams withdrew from the last Grand Slam of the year on Wednesday, revealing she is suffering from an autoimmune disease.
The former world number one quit the tournament before her second-round match against Germany's Sabine Lisicki then released a statement explaining the extent of her health problems.
"I'm really disappointed to have to withdraw from this year's US Open," she said in the statement.
"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."
Williams has been battling health problems for the past year. She retired from her third-round match at the Australian Open with a hip problem and did not play again until June.
The 31-year-old played only a handful of matches since making her comeback and had not played since Wimbledon, pulling out of recent tournaments in Toronto and Cincinnati. She only resumed practising last week because of what she said was an "energy-sucking" mystery illness
Williams won her opening clash with Vesna Dolonts of Russia 6-4, 6-3 on Monday but there were some alarming signs. There were times she looked to be lacking the zip that helped her win seven Grand Slam singles titles and her voice was hoarse.
She told reporters she had been diagnosed with an illness but did not say what it was until Wednesday.
"I enjoyed playing my first match here and wish I could continue but right now I am unable to," said Williams. "I am thankful I finally have a diagnosis and am now focussed on getting better and returning to the court soon."
Williams is one of the most accomplished players of her generation, winning Wimbledon five times and the US Open twice and playing in the final at the Australian and French Opens.
She has also won three Olympic gold medals and all four Grand Slam doubles titles with her younger sister Serena.
But Williams, whose achievements have been largely overshadowed by Serena's, has not won a Grand Slam title since 2008.
Speculation about her future has intensified over the past few months but earlier this week she confirmed she had no intention of retiring.
"I think she's a tough girl," Lisicki said. "I think she'll come back."
Sjogren's Syndrome is a chronic disease where white blood cells attack moisture-producing glands. The most common symptoms are dry eyes and dry mouth but the illness can also cause dysfunction of other organs and body systems.
Nine out of 10 patients are women and most experience extreme fatigue and joint pain.