Melbourne, Australia (SportsNetwork.com) - World No. 9 and reigning U.S. Open champion Marin Cilic has pulled out of this month's Australian Open due to a shoulder injury.

Cilic also sat out this week's tournament in Brisbane due to the injury.

"I am really disappointed to announce that I had to withdraw from the Australian Open," Cilic said on his Web site. "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."

Cilic won four events in 2014, including his first career Grand Slam title when he topped Kei Nishikori in the final at Flushing Meadows.