Federer, 37, first became No 1 in February 2004 after winning the Australian Open for the first time.
The Swiss remained on top of the rankings for 237 weeks, the longest ever consecutive run for one player, but was deposed in 2008 by a 22-year-old Nadal.
And Ferrero, himself a world No 1 back in 2003, reckons that having fellow Spaniard Carlos Moya as a role model, and now a coach, made a big difference too.
“I remember that he started very early competing with the best ones,” Ferrero told Spanish newspaper El Confidencial.
“At just 16 years old and when he turned 22 he had already a lot of experience.
“Having Moya next to him helped him a lot, as role model in all the training sessions, in matches.
“All this helped him to come as fast as possible at the top.
“I am convinced that overcoming Federer was a challenge and a goal that they set.
“Once they achieved it, which was something they saw as difficult, the satisfaction must have been big.
“Overcoming him also put Rafa up there to be shot at by others.
“It said: ‘Here I am and I will fight for it for a long time’.
“He fights in a better way, mentally he is able to suit to every situation.
“He finds immediate solutions.”
Nadal will defend his US Open title next week where Novak Djokovic will start as favourite and Federer will be the No 2 seed.
However, the Spaniard is still on top of the rankings and will likely retain that title until the end of the year.
But he will desperately want to close the gap on Federer: he currently has 17 Grand Slams to the Swiss’ 20.
And Ferrero knows he is unlikely to give up on that goal until he is forced to.
“You have many things that make Rafa different,” Ferrero added.
“From his game style to, especially, the mentality he has.
“Not giving up a point, from easily losing a match to comeback without never giving up.
“Keep fighting when the easiest thing would be to give up.
“People who did not play sports do not know how difficult this is.
“He feels frustrated, but frustration doesn’t catch him.
“That’s what marks the difference than the rest.
He also said: “What makes Rafa different than the rest is that he competes at a higher level.
“He is able to play every point at the maximum level, one after another without dropping in terms of intensity in any moment.
“And mentally he is able to adapt every situation that it is being put ahead of him in a very quick way. Mentally he is much higher than the others.”