Federer will be bidding to extend his record to nine Wimbledon crowns when the tournament begins next week, a little over a month shy of his 37th birthday.
Nadal won a record-extending 11th French Open title last month, shortly after turning 32.
Australia's Pat Cash, who won Wimbledon at the age of 22 in 1987 for his sole major title, told CNN Sport at Roland Garros he is amazed by their "freakish" motivation bordering on "obsession."
"I don't understand how he [Nadal] does it, I really don't know. It's some form of obsession, there is no doubt about it, I don't know what drives him, but it has to be," said Cash, who now coaches American Coco Vandeweghe.
"Federer as well ... Why would he be playing at the age of 36 other than to please your adoring fans, and because he likes playing and he likes to win titles?
"There is some obsession, there is some ego-driven thing that's really pushing them massively.
"Novak [Djokovic] is normal. Novak said, 'I lost motivation'. That is normal.
"In my era, the Tour wasn't as nice and as comfortable as it is now. We had to travel and go overseas and it wasn't exactly first-class seats. We almost all lost motivation.
"[Bjorn] Borg retired at 26, I lost interest at 24 or 25, [John] McEnroe has talked about it, [Andre] Agassi has talked about it. That's normal.
"What's not normal is Federer and Nadal. And I don't know what it is. Maybe to get better, maybe to win more.
"Whatever it happens to be, it's phenomenal. To me, it is kind of weird and most guys, the ex-players, they sort of look at them and go 'Why is Federer still playing?'
"Nadal, I think he feels like he's got something to prove, he's had a few years off, he's come back and he's had some motivation to get back again after injuries."
"They set the benchmark so ridiculously high that somebody like Novak looks like a failure compared to them because he's lost motivation for a year, which is ridiculous," added Cash.
"Novak had the best year , possibly the best year tennis has ever seen. He played Nadal at his peak and Federer at his peak and he beat them. And these are two of the greatest guys we are talking about, so that took a lot out of him.
"The same with Andy Murray. Murray had a great year  and then was exhausted the next year. That's normal.
"What's not normal is that they [Federer and Nadal] go back year in, year out. It really is freakish and I can't quite find the answer to that, to exactly what it is."
Can Nadal win Wimbledon again?
"I think he'll struggle on faster courts because of the position where he is standing. He did win the US Open [in 2017], but let's face it the draw was pretty favorable," said Cash.
"I think some of the bigger hitters will put him in a lot more trouble on a faster court. I'd like to see him come up.
"There is no reason why he shouldn't do well at Wimbledon, but he is not going to if he stands way back on his returns.
"He's not going to beat Federer with the tactics he's got now. Federer has beaten him the last five times they played.
"Federer stands really close to the baseline, so he is going to struggle against someone like that. But there are not many players like that.
"Nadal is super quick, and he gets to everything, and if you leave it a bit too high, you are in big trouble.
"But he does drop the ball short and you have to punish that."