Rafael Nadal has struggled for form on clay but the nine-time champion remains Roger Federer's favourite to win next month's French Open.

Nadal has failed to reach the final at his past two clay-court events, beaten by eventual champion and world number one Novak Djokovic in the semi-finals of the ATP World Tour Masters, while the Spaniard was a third-round casualty at last week's Barcelona Open.

However, Federer is not writing off clay-court specialist Nadal ahead of the season's second grand slam, which gets underway on May 25.

"At the French Open, Rafa for me is still the favourite alongside Djokovic who has been playing so well," said Federer, who headlines this week's inaugural Istanbul Open.

"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."

Federer won the 2009 French Open, while the Swiss veteran is a four-time runner-up to Nadal in Paris.

The 33-year-old is still finding his feet when it comes to this year's clay-court season.

"It's about getting used to the sliding, deciding how aggressive I want to play," Federer added.

"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."