It's hard to imagine, isn't it? Since the turn of the century, these two have developed a rivalry that is as good as any world sport has known. It's up there with Ali and Frazier, Nicklaus and Watson or Coe and Ovett. But are we witnessing the end of it?
Federer is still No.2 in the world, but his grip on that position is loosening. Andy Murray is only 365 world-ranking points behind and, if the Scot wins the title in Miami this week, he will take over the No.2 berth. If it doesn't happen this week, Murray will have another chance to do it at the Monte Carlo Masters next month as the Swiss will also be absent from this event.
Nadal briefly dropped to No.5 during his absence from the tour with chronic knee problems, but is now back in the top four.
Novak Djokovic is out of sight at the top so, if Murray claims second spot, it would mean that Federer and Nadal would be in opposite sides of the draw in grand slam tournaments for as long as that remained the case. Which, in turn, would mean that they would have to reach the final to play each other.
It would take a brave man to bet on both Murray and Djokovic failing to reach the final of a grand slam event considering the imperious form the pair have shown in recent months.
The fact that Federer and Nadal are divergent characters only adds to the appeal: Federer in his crisp, traditional tennis attire; Nadal with his neon-coloured, rugged look. Their mutual respect makes it even more charming; both are at pains to compliment the other at every turn.
The pair have played 29 times, with the Spaniard leading their head-to-heads 19-10. They have produced some truly remarkable matches over the years, the most memorable perhaps being the 2008 Wimbledon final which Nadal won 9-7 in the fifth set of an epic encounter which lasted nearly five hours and concluded in near darkness.
In reality, though, the spark of the Federer/Nadal rivalry has been fading for a few years now. In fact, they haven't produced a truly classic match since the 2009 Australian Open final, in which Nadal emerged as the victor in five sets and reduced Federer to tears.
When they played in the quarter-finals at Indian Wells a fortnight ago – it was the earliest stage of a tournament that the pair have met in almost a decade – Nadal won comfortably. Federer was hampered by a back injury but it may also have been an indication that the Swiss is picking up more niggles than he did in his 20s.
As a result of his increasing fragility, Federer will play a reduced schedule this year, a decision which is made easier by the fact that he is now exempt from every Masters 1000 event. He can skip them without penalty, if he wishes, due to his age, the number of years he has been on tour and the number of matches he has played, hence his absences from Miami and Monte Carlo.
Nadal has made a remarkable comeback from his injury lay-off, winning three of four tournaments he has contested, but there are still considerable question marks over the ability of the Spaniard's knees to hold up throughout the season, particularly on hard courts. It is unclear how many years he has left in him.
If this is indeed the end of the Federer/Nadal era then tennis is nevertheless in safe hands. Djokovic and Murray are developing an intriguing rivalry of their own but their similar styles mean that the magic of the Federer/Nadal pairing isn't quite there.
Federer and Nadal have an X-factor that is missing from every other player on the planet. It's been a joy to watch them fight it out over the past decade or so. So let's just hope that they've still got a little bit more left in them.