Venus Williams's participation in the Australian Open has been put in doubt after she withdrew from January's ASB Classic in Auckland, New Zealand, because of ongoing health issues.
The 31-year-old former world number one has been receiving treatment for Sjogren's syndrome.
The Australian Open, the first Grand Slam of the year, begins on 16 January.
"We are disappointed to have lost a player of her calibre," said Auckland tournament director Richard Palmer.
"While withdrawals of this nature are part and parcel of sport, we sincerely feel for the fans who will also be disappointed by the news of Venus's withdrawal."
The seven-time Grand Slam winner, who is currently ranked 103 in the world, had been handed a wildcard entry to the tournament after playing severeal exhibition matches in recent weeks.
However, the competition's website announced Williams had told her agent, Carlos Fleming, that she was not fit to take part despite recent exhibition match appearances.
"Only last month she was reported saying she was looking forward to her first tournament of the year in Auckland and had heard great things about the ASB Classic," added Palmer. "Her goal was to play a full schedule next year."
Williams withdrew from her second-round match at the US Open in August, citing problems with Sjogren's syndrome, an autoimmune condition which can cause fatigue and joint pain.
Speaking in September, she said: "Sjogren's is something you live with your whole life. The good news is now I know what's happening after spending years not knowing. I feel like I can get better and move on."
WHAT IS SJOGREN'S SYNDROME?
The body attacks its own moisture-producing glands, such as the tear and salivary glands. It can also affect the kidneys, liver, pancreas, lungs, blood vessels and brain