The Croatian had been due to take on Kenny De Schepper later today, but a knee problem has forced him to pull out of the competition.
"I would say this is a very black day. I mean, the other days, other weeks, there were no pull-outs. It's difficult to say what the real issue is," said the 24-year-old. "I just consider my own case. It's more because of obviously much lower bounces, putting more pressure on my body and my knees, as I'm pretty tall.
"It also has a difficulty on movement. It's a bit tougher to move on grass than other surfaces."
Cilic had been due to face Andy Murray in the fourth round.