The top seed was sent crashing out of the Cincinnati Masters after losing in straight sets in the third round.
Djokovic told reporters: "It's unfortunate, but it's more than obvious I'm not playing even close to what I'm supposed to play.
"I have to keep on working and trying to get better for the US Open. I just don't feel comfortable. That's it."
The Serbian will now head into the US Open on August 25 on back-to-back defeats after losing to Jo-Wilfred Tsonga in Toronto last week.
Thirty-two-year-old Robredo is currently ranked number 224 in the world; his highest ever ranking was number 16.