MELBOURNE - Seven-times grand slam champion Venus Williams has withdrawn from her Australian Open warm-up event in New Zealand due to "health reasons," placing her participation in the year's first grand slam in doubt.

American Williams, who has been receiving treatment for Sjogren's syndrome, an auto-immune disease, had been given a wild card for the Jan 2-7 Auckland Classic.

Venus Williams of the US serves to Flavia Pennetta of Italy during an exhibition tennis match in Milan, Dec 3, 2011. [Photo/Agencies]

"After playing a number of recent exhibition matches, Williams told her agent Carlos Fleming she was not ready to play in Auckland," a statement on the tournament's website said.

"We are disappointed to have lost a player of her calibre. While withdrawal 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," tournament director Richard Palmer added in the statement.

Williams withdrew from September's US Open with Sjogren's, a chronic disease where white blood cells attack moisture-producing glands and can cause dysfunction of organs and body systems.

The five-times Wimbledon champion, who changed her diet to include more vegetables after suffering fatigue from the disease, said last month that she hoped to return to the tour full-time in 2012 and to top the rankings.