Rafael Nadal and Roger Federer performed their compelling act a few times at Wimbledon, several more times in Paris, but also in Madrid, Shanghai, Dubai, Hamburg and Monaco. The rivalry between Nadal and Federer, considered the two best tennis players in the world, has been showcased all over the globe, but never in New York.
They have played 21 times since 2004, when they first met in Miami, — the only time they played in anything other than the final or semifinals of a tournament — and Nadal holds a 14-7 advantage. But somehow they have never met at the United States Open, where Federer has reached the last six finals and won five and where Nadal has never make it past the semifinals.
The rivalry may seem one-sided, especially on clay, but on center court at Arthur Ashe Stadium, Federer had been invincible until last year’s final, when he lost to Juan Martín del Potro. If Federer, who had won 41 consecutive matches at Arthur Ashe, can get back to the final, and Nadal overcomes his usual end-of-summer obstacles and reaches the Open final, it would provide the most compelling final matchup here since Pete Sampras beat Andre Agassi in 2002.
The friendly foes, Nadal and Federer, No. 1 against No. 2.

“People seem to love it,” Federer said. “It’s one of the biggest rivalries we have in all of sports, and I’m happy it’s been on a fair basis. It would be a bit of a pain if it weren’t like that.”

Mostly because of the respect Nadal showed for Federer when he eventually assumed his mantle as the world’s best player, Nadal never outwardly annoyed Federer, so their relationship has not had the edge that Jimmy Connors’s and John McEnroe’s or Justine Henin’s and Serena Williams’s had.

“Look, he’s not my rival since three weeks,” Federer said. “He’s been my rival for almost five years now. So we’re a very healthy rivalry. We get along really well. We chat a lot in the locker rooms, just hang out sometimes.
“So it’s nice to have a rival like Rafa who has been so respectful, really. I think I’ve helped him to be a better player. Same for me. I think he’s helped me to become a better player.”
Whether the speed of center court or Nadal’s late-season injuries kick in again to thwart the matchup, Nadal said he fully expected Federer to hold up his end.
“If I am in the last rounds, I am going to have a big chance to play against him,” Nadal said, “Because he always is here.”

Federer comes into the Open on an upswing after a difficult summer. After winning the Australian Open in January for his record 16th Grand Slam title, he did not win another tournament until Cincinnati last week. At the French Open, he lost to Robin Soderling in the quarterfinals, breaking his record of 23 consecutive Grand Slam semifinal appearances. After he lost to Tomas Berdych in the semifinals at Wimbledon, he uncharacteristically complained of leg and back problems.
He took a six-week vacation, but while in Toronto he was bothered by small problems in his arm, wrist, chest and shoulder, he said. He recovered to reach the final there and then won in Cincinnati. On Saturday, he proclaimed himself fit and ready to compete for a sixth title, whether it is against his friendly rival or not. He also said he was taking on a new coach, Paul Annacone, who had worked with Sampras.

“From the mental side, I have no problems whatsoever,” he said. “Physically, I feel the same thing. I feel as good as I can feel prior to a Grand Slam.”

Another article here:

Rafael Nadal and the U.S. Open: If not now, when?