Their first meeting came more than a decade ago in Miami.
Switzerland's elfin Roger Federer, lean -- almost frail to the naked eye -- and wearing a six-inch ponytail and a too-wide white Nike headband, was 22. Five years younger, Rafael Nadal, an absurdly muscular Spaniard, was garbed in his pirate gear of the day.
Federer had just won Indian Wells, defeating Tim Henman in the final, and was the world's No. 1-ranked player. He had also suffered some California sunstroke; after several days in bed, he arrived in Miami.
It wasn't close. On the final point, in a pattern that would be repeated again and again over the years, Nadal hit three successive left-handed forehands to Federer's right-handed backhand. The last led to a thundering overhead -- followed by a joyous knee-lifting, upper-cutting fist pump -- and a startling 6-3, 6-3 victory.
"I don't even remember what the score was exactly," Federer said back in March at this year's Sony Open, his best guess (6-3, 6-4 maybe?) missing the mark by a single game. "I never really got into the match. He played great, and I was impressed with what I saw. I mean, that was clearly for him a big arrival on the scene, because that's always a big deal if you can beat a world No. 1 on hard courts.
"I think everybody thought he was just a clay-courter, and he proved that he was probably going to be more than just that. [He] went on to have this great career many expected him to have."
And so the game, the fascinating elite arms race in men's tennis, was on.
Federer is considered by many to be the greatest player of all time, but he's lost 23 of 33 matches to Nadal.
For now, Federer is the all-time leader with 17 Grand Slam singles titles; Nadal is third, with 13.
They have met five times at the French Open, which begins Sunday in Paris, and Nadal has won all of those fascinating red-clay encounters. How would those major totals be different if Federer had managed to win one or two?
"I don't think you can crystal ball that," said Darren Cahill, former coach of Andre Agassi and Lleyton Hewitt and now an ESPN analyst. "There are so many factors: length of career, luck of the draw, the other players coming through. You've got guys like Novak Djokovic and Andy Murray and now [Stanislas] Wawrinka who think they have a chance to win when they walk out there.
"How many weeks did Roger spend at No. 1? How many consecutive Grand Slam semifinals did he make? You could write a line down the middle of a chalkboard and write all the accolades and achievements of each guy and you'd run out of space."
In the liquid, ever-shifting dynamic between two Grand Slam champions over their careers, each match at a major is charged with historic significance. Pete Sampras and Agassi had nine Grand Slam meetings. Sampras won six of those and five times went on to win the title -- a large part of the reason Sampras finished with 14 Grand Slam singles titles, seven more than Agassi.
Nadal is 9-2 against Federer in Slams, including eight finals. Incredibly, each of those nine wins was a piece of a major championship. If Federer had won two or three of his 0-for-8 run at the Australian Open and Roland Garros, he might own an insurmountable lead as he moves toward retirement.
Djokovic, who beat Rafa in the Rome final last week and is the contemporary who might well have the biggest hand in determining the final outcome in the category of major titles, smiled when asked in Miami if he thought Rafa might catch Roger.
"Well," Djokovic said, raising his eyebrows, "we'll see. We can't really predict what [the] future brings, but considering the results he had for only 27 years [of age], it's possible. Definitely."
The all-out fury with which Nadal plays has always left him physically at risk. Over the years, his knees have demonstrated a difficulty in supporting the power and torque of his game. But, suddenly, it is his head that seems to be holding him back.
"I started the season OK," Nadal said," except for some lack of confidence and competitiveness in important moments of certain matches."
Oh, is that all?
For the first time in a decade, Rafa exited April without having won a title. If he loses in Rome or Paris, he'll have three losses on clay in the same year for the first time since 2004. Even winning Madrid raised questions because the title came via the retirement of increasingly fragile Kei Nishikori.
Forgive Federer if he seems a little preoccupied at this year's French. He pulled out of his first match in Madrid a few weeks ago to be with his wife, Mirka, in Switzerland when she delivered their second set of twins, Leo and Lenny. The hashtag on Federer's Twitter announcement: #TwinsAgain#Miracle.
Rafa, meanwhile, has won 59 of 60 matches at Roland Garros -- the best record for any player at any Grand Slam. That translates to a total of eight French Open titles, accrued prior to one week after his 27th birthday, and one more than the seven Wimbledon championships of Federer and Sampras.
Incredibly, there might have been more.
Nadal qualified for the 2003 Roland Garros main draw at the age of 16 but suffered an elbow injury in practice. A few weeks later, just past his 17th birthday, he became the youngest man to reach the third round of Wimbledon since Boris Becker in 1984. A stress fracture in his left ankle, suffered in Estoril, forced him to miss the 2004 French Open.
In the 2005 semifinals, with Federer looking for the career Grand Slam, Nadal defeated his Swiss opponent 6-3, 4-6, 6-4, 6-3.
"I'm sorry for you," Nadal said at net.
"No, no, you played well," Federer replied. "Good luck for the final. Good luck for the future."
Nadal lost the first set of that final to Mariano Puerta but won his first French Open title comfortably in four sets.
"These moments are moments when everything comes to you," Rafa said afterward. "For the first time, I cried after winning a match."
Nick Bollettieri, who will be enshrined at the International Tennis Hall of Fame this summer, was impressed.
"Holy [----]!" he remembered recently. "Rafa's got that extreme Western grip and the wide-open stance. Great backhand. You see him winning from so far behind the baseline at first. That's a perfect recipe for winning on clay."
In the 2006 final at Roland Garros, Federer won the first set but fell again in four. A month later at Wimbledon, Nadal made his first final but lost to Federer. It was the same scenario in 2007, but Nadal took Federer to five sets, foreshadowing what would happen in 2008. After allowing Federer a total of only four games in the French final, they played one of the greatest matches of all time. Nadal prevailed 9-7 in the fifth set.
"The kid changed his game," Bollettieri said. "By standing closer to the baseline, coming forward and hitting the ball earlier, flattening out his forehand, he solved the riddle of playing on grass. With that heavy cross-court forehand, he's going to beat a one-handed backhand most of the time."
Nadal would handle Federer in the 2009 Australian Open final and again in the 2011 French Open final and 2012 Australian Open semifinals. Last year in Paris, Federer lost to Jo-Wilfried Tsonga in the quarterfinals; Rafa, coming back after a seven-month injury sabbatical, won the title and ran his Grand Slam singles total to 12.
"Of course," Nadal said after taking his eighth French, "winning 17 Grand Slam titles, that's miles away from me. I'm not even thinking about it."
Who will reign supreme?
Even with his peerless record at Roland Garros, Nadal has always had the capacity to surprise.
After missing seven months while tending to his ailing knees, Nadal came back last year and, toward the end, won three consecutive hard-court events in North America. The win at the US Open (he beat Djokovic in the final) gave him 13 Grand Slam singles titles.
Federer was Nadal's victim in the semifinals.
"I think Rafa played well," said Federer, who was disappointed in the way he served. "I think he's the way I know him. He's played me this way many times."
This year in Australia, it looked for all the world that Nadal would hit 14. But in the final, Stanislas Wawrinka played like his hair was on fire and, quite suddenly, found himself a major player.
"It's not only about him [Nadal] and Roger," Djokovic said in Miami. "There are other players that are able to win Grand Slams. [Look at] Stanislas Wawrinka. Something different is happening in our sport. We cannot underestimate or forget also the other players aside from the top four."
Djokovic didn't say it exactly, but the structure and tone of his answer suggested he thought Nadal has a chance to catch Federer.
"Even though you look at Rafa's game five years ago, you would think he has not much to improve, but he keeps finding the little edges and room for improvements," Djokovic said. "He's a hard worker. He's somebody that is very committed to the sport, and that's why he has had so much success."
Recently, Agassi jumped into the fray, saying Nadal is the best player of all time.
"I'd put Nadal No. 1, Federer No. 2," Agassi told Singapore's Straits Times. "Nadal had to deal with Federer, Djokovic and Murray in the golden age of tennis. He has done what he has done, and he's not done yet."
Let's do the math: Nadal turns 28 on June 3, about the time he's usually closing in on the French Open title. Federer has reached four major finals since turning 28 and won two. Rafa's knees are the wild card here, but most experts queried were willing to grant him two, maybe three, more French Open crowns and pointed to his performance in the other Grand Slams as the key to the final tally. Most of them believe Federer might have one more in him, particularly the next two years at Wimbledon.
Nadal smiled after his most recent US Open title when the inevitable questions of running down Federer came up.
"Let me enjoy today," Rafa said. "For me, is much more than what I ever thought, what I ever dreamed. Only thing I can say is the same like I do every time: I'm going to keep working hard.
"You never know when [winning majors] start, when that finish, but 13 is an amazing number."