ISTANBUL: Switzerland's 17-times Grand Slam winner Roger Federer on Monday said he still believed Rafael Nadal was the favourite to win the French Open next month despite the Spanish star's troubled start to the clay court season.

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.

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'

Federer, who celebrated his arrival in Istanbul by thumping a ball into the Bosphorus that divides Europe and Asia, is the star attraction in the Turkish mega city along with the Bulgarian world number 11 Grigor Dimitrov.

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.

Federer, who has played two impromptu practice sessions with Dimitrov since arriving at the weekend, paid tribute to the Bulgarian as having a game “similar to mine” -- athletic with a one-handed backhand.

But Federer said his own experience showed now was the time for Dimitrov, 23, to knuckle down and go into “overdrive” to break into the world's elite.

“He (Dimitrov) has improved a lot in the past few years. I still believe he has a lot of improvements to make.

“He has just got to take the right decisions in the next few months and years on how exactly is he going to crack the top five, because it's a big step.”

“He needs to put in extra effort now and just go into overdrive,” said Federer.

Musing on his own long career, Federer said he had taken the right decisions at the right time, especially when he first became number one in 2004.

“Number one was a big deal and I decided I wanted more of it... I am happy I took that decision otherwise I would have many regrets. I just have minor ones.”