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

The U.S. Open champion says on Friday, "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."

The Zagreb Indoors starts on Monday. Cilic won the tournament in 2009, 2010, 2013 and 2014.