Marin Cilic will be unable to win the Zagreb Indoors for a fifth time in seven years after withdrawing from the ATP 250 event with a shoulder problem.
Cilic won back-to-back finals in 2009 and 2010, as well as each of the last two tournaments, defeating Jurgen Melzer in 2013 and then Tommy Haas in last year's showpiece.
However, the world number nine announced on Friday he will not feature at the 2015 edition, set to get under way next week, as he continues to struggle with injury.
"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," he said.
The US Open champion has already missed the Brisbane International and the Australian Open this year due to injury.