Novak Djokovic currently heads the standings, with Nadal just over 2,000 points behind and defending US Open champion Andy Murray a close third.
But Nadal's lengthy absence post-Wimbledon last year, which saw him miss both the US Open and Australian Open, means he has no points to defend until Feburary.
Djokovic, on the other hand, reached the final 12 months ago and, with 2,000 points on offer for the winner, Nadal could even overtake his rival in New York.
Nadal said: "If I am number one, it will be an amazing, amazing season for me. If I am not number one, it still will be an amazing season for me.
"This tournament is going to make a big difference. After here I will say not everything will be decided, but things can be more clear or completely less clear."
What is clear is the Spaniard goes into the tournament as the form player after an astonishing season on what was considered his weakest surface.
Nadal, the 2010 champion in New York, has played in three Masters series events on hard courts and won the lot, most recently back to back in Montreal and Cincinnati.
Add to that six clay-court titles, including an eighth French Open, with the only blip a first-round loss to Steve Darcis at Wimbledon that had left pundits wondering whether he would even make the US Open.
But he arrived in North America rested and healthy, with no strapping on his troublesome knee and moving superbly.
Nadal's victims have included Djokovic and Roger Federer, both high-quality matches decided on tie-breaks, and the Spaniard was typically keen to play down their importance.
Djokovic feels Nadal is playing more aggressively, but the Spaniard said: "I think you can play aggressive when you are playing well.
"It's true I'm trying to take the ball a little bit earlier, that I worked on my game. But it's true that in the past when I was playing well on this surface I had good success. So it's not something crazy that changed today.
"It was a very tough match against Novak in Montreal - anyone can win the match. Then I had a very tough one against Roger in Cincinnati. Anyone can win that match. It was lucky that I won both."
Nadal and Federer could meet in the quarter-finals, with the Swiss seeded only seventh.
Age appears finally to be catching up with Federer, but the 32-year-old made it very clear he is not in New York to make up the numbers.
He said: "I think it's an exciting draw with Rafa being nearby. Plus we have never played here. I really hope that I can make it.
"But clearly when I come here I don't just look at trying to make the quarters. I'm clearly here trying to win the tournament."
Federer has won only one tournament this year, the pre-Wimbledon event in Halle, while his proud record of consecutive grand slam quarter-finals ended at 36 with a second-round loss to Sergiy Stakhovsky at the All England Club.
He tested out a new racket with a bigger head in clay tournaments in Hamburg and Gstaad but struggled with his sometimes troublesome back and lost to unheralded players both times.
Federer has postponed further testing of the racket until after the US Open, and the form he showed against Nadal in Cincinnati had more than a little of the old magic.
The 32-year-old is mostly just happy to no longer be worrying about his back.
He said: "At times I was playing a lot having it in the back of my mind, and that has definitely affected me sometimes with my movement, just trusting the movement, trusting my play, and actually not being able to really focus on the point-for-point mentality that you want to have.
"So now I can really say I'm just focused on the point for point. I'm happy that I'm playing well again. My confidence is back."
Federer was eager to stress, meanwhile, that dropping to seventh in the rankings was not that big a deal.
"I don't think it's a huge drop from number four.," he said. "What's important is that I concentrate on my game and that the passion is there, that I work the right way, that I'm prepared, and then that I feel like I can win a tournament.
"But I have looked at the rankings my whole life. I used to be incredibly excited on a Monday seeing how many spots my ranking went up or down. Usually I was more excited that it was going up.
"The older you get, the less you pay attention to it. But, nevertheless, clearly I want to move up from here. I only have the quarter-finals to defend, so I hope I can add some points to the rankings."
Federer and Nadal are both in action on day one, with the Spaniard meeting American Ryan Harrison before Federer takes on Slovenia's Grega Zemlja in the night session on Arthur Ashe.
Fourth seed David Ferrer begins his campaign against 18-year-old Australian qualifier Nick Kyrgios on Louis Armstrong.