The 39-year-old said the condition was exacerbated by the heat and humidity in Ohio as he visibly struggled in a 6-2 4-6 4-6 loss to Argentina's world No 50 Thiago Agustin Tirante.
Djokovic, currently world No 5 and playing his first match since losing to Jannik Sinner in the Wimbledon semi-finals in July, slumped to his hands and knees before taking a medical timeout midway through the second set.
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He told reporters: "Congrats to my opponent. It was not an enjoyable match for me, for sure. The way I felt… Not the first and I guess not the last time, but it is what it is."
The 24-time grand slam champion added: "It's just a condition that I have, health-wise, that has been bothering me for the past couple of years.
"[It causes] a lot of issues, especially when it is humid and it is hot.
"I did anticipate it, but there are all these things, like nerves and everything involved that make it worse, and that's what happened."
When asked if he would be returning to Cincinnati next year, three-time winner Djokovic said: "I certainly hope so, but it looks more likely not, unfortunately. But let's see what the future brings."
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