US Open champion Marin Cilic will miss the Australian Open after failing to overcome a right shoulder injury.
The world number nine from Croatia made the decision after being advised by doctors that playing with the injury could lead to an even longer lay-off.
Cilic's withdrawal follows that of France's Jo-Wilfried Tsonga, who pulled out with an arm injury.
Both players took part in the inaugural International Premier Tennis League at the end of the year despite their problems.
Cilic said in a statement on his official website: "I am really disappointed to announce that I had to withdraw from the Australian Open. Right shoulder injury, that hampers me for some time now and forced me to withdraw from the ATP event in Brisbane, is improving day by day, but I still don't feel fit enough to be able to compete at the highest level.
"I am really sad that I won't be able to participate in Melbourne, especially after winning the last grand slam in New York, but I was advised by doctors to be careful as much as possible to avoid even more serious problems.”