Djokovic suffered a 7-6, 4-6, 6-1 loss to the Japanese in a second round tie.
It was the Serbian's first tournament after undergoing an elbow operation, which ended his Australian Open challenge in January.
"For me, it felt like the first match I ever played on the tour," said Djokovic post his defeat.
He added, "Very weird. I mean, I just completely lost rhythm, everything. Just struggled also a little bit with the health the last couple of weeks."
"I was not even supposed to be here because of the surgery that was only five, six weeks ago. But I recovered very quickly, and I got myself ready."
Meanwhile, world number one Roger Federer has advanced to the third round of the competition with a 6-3, 7-6, victory over Argentina's Federico Delbonis. (ANI)