Former world number one Novak Djokovic has withdrawn from next week's Qatar Open tennis tournament. He now faces a race to be fit for the first grand slam of the year, the Australian Open.The 30-year-old Serb had been scheduled to start the new season at an exhibition match in Abu Dhabi yesterday when an elbow injury flared up. Djokovic said in a statement that the situation with the elbow has not changed for better and he was still feeling the pain. The 12-time grand slam winner has not played since he was forced to retire against Tomas Berdych in the quarter-finals of Wimbledon in July due to an elbow injury.Meanwhile Japan's Kei Nishikori has pulled out of next month's Sydney International, putting his comeback on hold from a wrist injury that has kept him out of action for more than four months.The withdrawals of Djokovic and Nishikori raise further doubts about their participation in the Australian Open, which begins on January 15.