That is the view of Switzerland’s Fed Cup captain Heinz Gunthardt.
Federer will make his return to clay next month at the Madrid Masters after a three-year absence.
The 37-year-old has begun familiarising himself on the surface this week while the rest of the tour has been in Monte Carlo for the first European Masters 1000 event of the year.
Unsurprisingly, Nadal is the favourite to dominate the clay campaign and win a 12th title at Roland Garros in June.
But the Spaniard has had an injury-hit year and with a busy schedule ahead, he could breakdown again.
There is also the Novak Djokovic factor with the Serbian aiming to be the first man to hold all four Grand Slams at the same time on two different occasions.
The question of whether Federer can mount a challenge is unclear but Gunthardt feels the 20-time Grand Slam champion is capable if conditions suit him.
However, he also makes clear that Nadal will be untouchable if the 32-year-old is at his best and healthy.
Gunthardt told Swiss news outlet Keystone-SDA: ”It’s going to be very interesting.
“It also depends on the circumstances, clay can be very different.
“If it rains in Paris, the place can be very slow. There can then be holes, the ball is no longer 100 per cent perfect and it will be much harder to hit the ball early.
“Then Federer would have to play further back, then I do not see it that way.
“But if it stays dry and hot, center court at Roland Garros can be like a hard court with a bit of clay on it.
“Then it may be that he hardly has to change his game and absolutely has opportunities.
“In Paris, there is just one person who stands above all others, and that’s Rafael Nadal.
“If he is fit and plays his best tennis, then he is unbeatable on clay.”