Rafael Nadal is the yin to Roger Federer's yang — and not just on the tennis court where their styles could not be more disparate.
Nadal's off-court comments at this US Open have been diametrically opposed to those of Federer on several topics, revealing the Spaniard's willingness to criticize where he finds fault, whereas Federer appears more happy-go-lucky about mostly everything.
Wikipedia describes yin and yang as "the combination or fusion of the two cosmic forces." In the tennis world, Nadal, 31, and Federer, 36, definitely qualify as opposing cosmic forces.
First, when Nadal, the No. 1 seed, and Federer, the No. 3, appeared in the same half of the draw, Nadal was quick to say that he wasn't looking forward to playing his arch-rival, to whom he has lost the last four times, including in this year's Australian Open final.
“If I am in semifinals, no,” said Nadal, who won here in 2010 and ’13. “I prefer to play against another one. It's obvious, no?"
It was a refreshing and candid answer from a man who is currently No. 1 in the world and the owner of 15 Grand Slam titles.
Federer, the owner of the all-time record 19 Slam crowns, by contrast, said he would "love" the opportunity to face Nadal in the semis.
On the topic of noise inside Arthur Ashe Stadium, which had the roof closed for his straight-sets win over Dusan Lajovic of Serbia on Tuesday, Nadal isn't a fan, either.
"Under the roof, the noise stays a lot inside," he said. "So that's to fix a little bit. The USTA make an amazing improvement for the fans, for the players, for the TVs, for everybody with the roof, and now they can make it even a little bit better if they can control a little bit more that."
He played under the closed roof on Tuesday night but had to go five sets against 19-year-old American Frances Tiafoe. Perhaps predictably, Federer said he appreciated the crowd noise, which got louder as the match got longer.
"When the crowd really got into it, it was really cool," he said. "It was a great atmosphere. It was loud. It was louder than what I'm really used to. Not that it wasn't loud before, but I think it comes back down on the court."
You can bet it will be plenty loud inside Ashe if these two legends continue to win and ultimately square off in the semifinals on Sept. 8.
Both had a day off on Wednesday after playing under the roof, while their would-be opponents will have to play back-to-back days on Wednesday and Thursday because of the rain on Tuesday.
On the court, the two cosmic tennis forces could not be more different.
Federer plays a free-flowing, elegant game in which he seems to float about the court at times. That style of play has kept him mostly injury free for most of his career. In recent years, he suffered a knee injury while bathing his daughters in the tub and his back has flared up at times, most recently causing him to withdraw from Cincinnati two weeks ago.
Nadal, by contrast, hits with such force and torque that he puts great strain on his body, which has contributed to several injuries across his career.
"Every ball [by Federer] is lathered with gratuitous action, spin for spin’s sake, spin as slapstick, and unlike Nadal, who rips violently upward on his shots to impart an ungodly number of rotations per second to the ball, Federer luxuriantly massages every shot as if to prolong the moment of impact and better feel the racket head moving over the ball, string by string," wrote Peter de Jonge in The New York Times Magazine.
What the two have in common, especially at this stage, is that they're both still playing championship level tennis into their thirties.
They have combined to win all three Grand Slam titles in 2017 — with Nadal winning his 15th major and 10th French Open title in June — and are the two favorites here in Flushing Meadows.
Yet even the way they talk about the current state of their games represents their yin and yang.
Federer has openly embraced talk that he may be playing some of the best tennis of his career despite being a married father of four while Nadal openly yearns for younger days.
Nadal joked that "everything is easier" in your twenties, adding: "And today, here I am at 31. If you tell me I will be here with 31, being No. 1 in the world, especially, seven, six, 10 years ago, I will not believe you, so I try to enjoy every day without thinking much about happened or what can happen."
Federer, of course, has nothing but praise for Nadal's game.
"Rafa's year has been exceptional, winning the 10th French Open," he said. "I mean, even people didn't think he was going to win the French Open again. For me, only once he retires I believe he won't win anymore. He's that good of a player."
Follow Adam Zagoria on Twitter @AdamZagoria for updates throughout the U.S. Open.