Federer, who took his sole French Open title in 2009, this week is refining his own clay court game at the Istanbul Open, the first ATP World Tour event to be held in Turkey and the first time that Federer has visited the country.
But Nadal, who with the exception of Federer's win has taken every French Open since 2005 to make nine in total, has shown a dip in form over the past weeks, failing to get past the third round at last week's Barcelona Open.
"At the French Open, Rafa for me is still the favourite alongside Djokovic who has been playing so well.
"Even though his (Nadal's) form is not as good as in previous years, I still believe when the French Open rolls around he is going to be very difficult to beat," he told a news conference in Istanbul.
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Federer, 33, is himself looking for his own form on the slower clay after a long hard court season, having lost in the third round to Gael Monfils at the Monte Carlo Masters this month. But he said the Istanbul event was ideal for his preparations.
"I have been on hard courts for seven-eight months so it needs some adjustments. My game is going to get better and better as we move along."
Federer, who so far has been unable to add to his sole French Open win in 2009 and won his last grand slam at Wimbledon in 2012, acknowledged that this year's French Open "is clearly a big goal of mine."
"But to play well at the French Open I need confidence. I hope that I can pick that up in Istanbul."
With Federer seeded one and Dimitrov two, the pair are slated to meet in the final in Istanbul. After receiving byes for the first round they should begin their campaigns from Wednesday.