Since winning his ninth French Open title in 2014, Rafael Nadal has won just one tournament—an ATP 250 in Buenos Aires. In that time, Novak Djokovic has won eight titles, including the clay-court Monte Carlo Masters, where he defeated Nadal the semifinals. But Roger Federer believes the Spaniard remains the favorite at Roland Garros.

“At the French Open, Rafa for me is still the favorite alongside Djokovic who has been playing so well,” said Federer, who won the French Open in 2009, the only year Nadal has lost at Roland Garros. “Even though [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 is in Istanbul this week, playing the ATP 250 clay-court tournament there for the first time. The 33-year-old lost to Gael Monfils in the Monte Carlo round of 16 two weeks ago; the last time Federer won a tournament on clay was in 2012, in Madrid.

“It's about getting used to the sliding, deciding how aggressive I want to play,” Federer said about playing on clay. “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.”