The defending champion continued his bid for the title with a solid performance inside the Arthur Ashe Stadium and has yet to drop a set in the tournament.
However, the Serb admits that he was pushed hard by Del Potro during the entertaining contest on Thursday night in New York.
"I love playing night sessions," said the 25-year-old. "We played some incredible rallies and some incredible points.
"I was lucky in the second set to come out with a two-set lead. Even though it was a straight-set win, it was closer than the score indicated."