Former world number one Novak Djokovic withdrew from next week's Qatar Open on Saturday and faces a race to be fit for the first grand slam of the year.
The 30-year-old Serb had been scheduled to start the new season at an exhibition match in Abu Dhabi on Friday when an elbow injury flared up.
"Unfortunately, the situation with the elbow has not changed for better since yesterday," Djokovic said in a statement.
"I still feel the pain. Therefore, I will have to withdraw from ATP tournament in Doha.
"Only when I'm 100 percent ready to play, will I be able to come back. I hope it will be soon. I want to thank everyone for patience and understanding."
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.
"Unfortunately Kei Nishikori has withdrawn from #SydneyTennis. We wish him well and hope to see him in Sydney in 2019," the tournament organizers posted on Twitter on Saturday.
The withdrawals of Djokovic and Nishikori raise further doubts about their participation in the Australian Open, which begins on Jan. 15.
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