The Serbian, struggling with an ankle injury, was on court for over two hours for the second day running, but managed to battle through for a place in the quarter-finals.
"It was another big test obviously, again a two-hour match," Djokovic told reporters. "Obviously I'm not physically out there feeling my best. Somehow feels that I always miss that half a step. But I'm trying. I'm fighting.
"After I lost the first set today, I tried to find that inner strength. I found it again. So that's a positive thing.
"Every day brings a new challenge and we'll see what happens tomorrow."