Seven-time Grand Slam champion Venus Williams has pulled out of an Australian Open warm-up tournament in New Zealand as she continues to battle a debilitating illness.Williams is suffering from Sjogren’s syndrome, an autoimmune disease, and after playing a number of recent exhibition matches she announced on Tuesday she was not ready to play in the Auckland Classic, which starts on Jan. 2.“We are disappointed to have lost a player of her calibre,” tournament director Richard Palmer said.“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.”Williams, whose ranking has slipped to 102 in the world, has described her medical condition as affecting her energy, causing fatigue and joint pains.Germany’s Sabine Lisicki, is top seed in a field that includes world top 20 players.