Good guy Federer.Federer said he believed that Nadal was still the man to beat on the red clay of Roland Garros, along with the in-form Serbian world number one Novak Djokovic.
"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.
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.
"It's about getting used to the sliding, deciding how aggressive I want to play," said the world number two.
"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."
- 'Go into overdrive' -