(CNN) -- Seven-time grand slam champion Venus Williams is in doubt for January's Australian Open after the American pulled out of her planned comeback at the ASB Classic in New Zealand.
The event was set to serve as the former world number one's warm-up for the tennis season's first grand slam, but Williams has withdrawn due to ongoing health issues.
"We are disappointed to have lost a player of her caliber," director Richard Palmer told the Auckland tournament's official website.
"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' withdrawal."
Williams has been receiving treatment for Sjogren's syndrome, an autoimmune disorder which affects energy levels and causes pain in the joints.
It forced the 31-year-old to withdraw from the second round of U.S. Open at the end of August, and restricted Williams to only 11 competitive matches in 2011.
Now ranked 102nd, she was given a wildcard entry to the Auckland event after playing recent exhibition matches, including one against younger sister Serena.
"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. Her goal was to play a full schedule next year," Palmer said.
Five times a champion at Wimbledon and twice a winner of the U.S. Open, Williams was beaten by Serena in her only Australian Open final appearance in 2003.
The Melbourne event starts on January 16, while the ASB Classic begins on January 2.
Germany's world No. 15 Sabine Lisicki will be the top seed in Auckland followed by fellow top-20 players Peng Shuai of China, Russia's Svetlana Kuznetsova and Italian Flavia Pennetta.
Hungary's Greta Arn will defend her title, while beaten finalist Yanina Wickmayer of Belgium also returns along with Germany's Julia Gorges -- who reached the 2011 semis with Peng.