The Mallorcan, a two-time winner at Flushing Meadows who last month claimed top spot in the ATP world rankings for the first time since July 2014, dropped the first set in his round of 64 meeting with 121st-ranked Japanese right-hander Taro Daniel and had to come from behind to seal a 4-6 6-3 6-2 6-2 victory.
The disappointing manner of his performance at Arthur Ashe Stadium may have led to concerns that Nadal, now 31, might be feeling the pace of a relentless and busy 2017 campaign.
"I don't feel fatigue. I feel positive," Nadal told reporters after his rather unconvincing four-set defeat of Daniel. "I feel hungry to keep playing. I'm excited for being here.
"Not tired at all. Is the last big, big event of the year. I want to try all the things that are in my hands to give me a chance. That's what I am doing: working with the right intensity, with the right attitude, then in the match trying to do the same.
"First two matches have been, of course, not great, but I am in third round. That's the most important thing. I have tomorrow practice. After tomorrow another important match for me. So I need to be ready for it."
Nadal was satisfied with his comeback against Daniel, but identified one key area in which he must improve to have a chance of winning a 16th Grand Slam singles title overall and a first at the US Open since 2013.
"Today, in my opinion, I saw it on the video screen, I hit around 40 winners," he added. "That's not a bad number. But 35 unforced errors, that's a bad number for me.
"I need to fix that. I need to improve that. I need to play with the feeling that when I'm hitting the ball, I'm not going to miss and with high intensity. That is the way I need to play if I want to have chances of success here. I going to work to make that happen."
Nadal's old nemesis Roger Federer has also struggled to find his best form in America following a back problem and has been taken to five sets in consecutive matches by teenager Frances Tiafoe and veteran Mikhail Youzhny. The duo are in the same half of the draw and could meet in the semi-finals, provided they both make it that far after less than stellar opening displays.