That is according to former world No 1 Ken Rosewall.
Federer boasts 20 Grand Slam titles - four more than Nadal - but at 36 years of age may not earn too many more.
Nadal, five years Federer’s senior, is favourite to win the French Open this summer.
The Spaniard has returned to his best after being dogged by injury problems over the past few years.
He will defend his French Open crown this month and Rosewall - a two-time Roland Garros champion - reckons Rafael Nadal can eventually overtake Federer’s tally.
"There's a good chance he will (catch Federer)... I think Rafa's going to have to decide that he won't play quite so much," Rosewall told Reuters.
"He's probably got to realise that he's got to save himself at some other time on some other surface, so he may not play on hard court tournaments so much.
"That aside, there's no reason why he can't win a number more big tournaments."
Roger Federer will sit out of the French Open this summer in preparation for Wimbledon.
The Swiss legend won the title on SW19 for an eighth time last year.
Rosewall, now 83, retains the record for the oldest ever men’s singles Grand Slam champion, having won the Australian Open at the age of 37 back in 1972.
And he concedes Federer, who turns 37 in August, could easily break that record.