Haas, whose last match came in the first round of the French Open on May 27, 2014, admitted that his recovery from shoulder surgery is taking longer than expected.
"At the moment it is not a mental thing, it’s more of a shoulder problem," he said at the BMW Open in Munich. "My shoulder is not quite there, where I want it to be in order to compete at a very high level and to give myself a chance to win some matches.
"So I have to postpone my comeback a bit and hope that I will be ready for the grass. Once I get back, hopefully my shoulder will allow me to play some more tennis."