ZAGREB, Croatia -- Defending champion Marin Cilic says he is withdrawing from the Zagreb Indoors because of the lingering right shoulder injury that also forced him to miss the Brisbane International and Australian Open.

"My injury is improving, but my arm still has not healed enough, so I will have to continue with the treatment for a while longer," Cilic said Friday.

The Zagreb Indoors starts Monday. Cilic, who won the US Open last year, won the event four times -- in 2009, 2010, 2013 and 2014.