Venus Williams has been forced to pull out of the Australian Open, which starts on Monday, through illness.
The former world number one revealed that she was suffering with Sjogren's syndrome in September last year.
A statement on her official website said: "I regret to announce that I am withdrawing from the 2012 Australian Open.
"After several months of training and treatment, I am making steady progress to top competitive form."
The 31-year-old previously pulled out of January's ASB Classic in Auckland but was hopeful of being fit for the first grand slam of the year.
The statement continued: "My diet and fitness regimen have allowed me to make great strides in terms of my health and I am very close to being ready to return to WTA competition.
"I have every intention to return to the circuit in February."
The auto-immune disease, which can cause fatigue and joint pain, caused her to pull out of last year's US Open.
She then announced that she was suffering with the ongoing medical condition, for which there is no known cure.
Williams has won seven grand slam singles titles, including five at Wimbledon.
Her only past success in Melbourne has come in the doubles, where she has won the ladies title four times with sister Serena and the mixed doubles once in 1998 when she partnered Justin Gimelstob.
World number one Caroline Wozniacki, a former doubles partner of Williams, said the American's health must come first.
"She's been unlucky with the virus thing," Wozniacki said. "I'm not completely sure what it is exactly, but the most important thing is the health.
"Tennis, it's a game. I'm sure she'll come back and fight and try to come back to the top again. But most of all, the most important thing is that you're healthy.
Hopefully she'll be 100 percent healthy by February."