The defending champion held a 6-4 6-1 3-1 lead when his opponent bowed out due to an unspecified illness.
"I'm happy with my performances. I played well against a quality opponent and I wish him a quick recovery," said Djokovic.
"It was a difficult situation for both of us with the rain yesterday. We were here for 10 or 11 hours, warmed up and then stopped. It was hard to get ready for a match that way," he explained.
Djokovic emerged as US Open champion last year, defeating Rafael Nadal in the final.