Federer has won a record 16 Grand Slams, seven more than Nadal. Yet Nadal has a 14-7 record against Federer. Nadal has won the Davis Cup title with Spain and an Olympic singles gold medal -- two achievements that have eluded Federer.
McEnroe, speaking at a sports conference in Doha, said, "There is an argument to be made that Rafael Nadal may be the greatest player eventually, even possibly now."
McEnroe adds that the 29-year-old Swiss star could add a "couple more" titles to that tally.
Former Swedish great Bjorn Borg said Federer is still the greatest, although "Rafa has the chance to be the greatest player" if he stays healthy.