In short, Federer thinks he is far too good to win on clay.
Here are excerpts from the Independent of Federer's explanation for not achieving on clay:
Federer: 'On clay, you don't need a volley or a serve. It's too easy'
Roger Federer doesn't want to offend Rafael Nadal, the man he acknowledges as the king of clay, but the world No 1 believes that winning the French Open can be less than a full test of a man's tennis.
"On clay, you don't need to have a volley," Federer says. "You almost don't need to have a serve. All you need to have his legs, an incredible forehand and backhand and to run things down. I'm not trying to take anything away from Rafa because he's an exception and he did everything on other surfaces as well, but I think you can get away with having problems with your game on clay more than you can on other surfaces.
"On a hard court, you can lure a guy in and do many more things. You almost have to have more feel. On clay, I don't want to say it's too simple, that you just have to keep the ball in court and wait for a mistake, but sometimes it's too easy."
"I don't think my problem was clay, even though there were lots of people who said that clay wasn't my best surface," Federer says. "Of course it's not, because I've won Wimbledon six times and the US Open five times in a row, but I'd always thought I could win the French Open. My problem, of course, was Rafa. The guy is unbelievable on clay. Some people don't want to believe it, but unfortunately, that's the truth for a big generation of players on clay, because I can also play on clay and so can [Novak] Djokovic, so there isn't much room at the top."
Clay places huge demands on a player's stamina. Because the ball comes off the surface more slowly than on a hard or grass court, it is much harder to hit winners, making the rallies noticeably longer. Coming to the net with the intention of finishing off a point more quickly can be perilous because it is harder to hit a damaging approach shot and you can become a sitting target for the opponent firing bullets at you from the baseline.
A different mindset is therefore required: you need to be patient and prepared to hit several shots to create a winning position rather than one or two. That is a particular challenge for a naturally attacking player like Federer. "The reason why clay has not been so easy for me is that on the other surfaces I can play my game without thinking," Federer said. "Everything happens naturally. I can turn defence to offence when I want to and how I want to. When I play well I know I can dominate players."
Federer admits it has taken him time to find the best way to play on clay. "I had to learn how to control my aggression because I love to finish points quickly. On hard courts and grass I love to play aggressively [and win points] in a couple of shots. That's the way we play.
"On clay, it's not that easy. You can do it on 50 percent of the points, but the other 50 percent you'll just donate to your opponent because you'll be taking too many chances. I had to learn how to play from far back in the court and to use the angles better when to attack. It was more of a geometry lesson for me.
"The more I played on clay the more I started to understand the game on clay, even though I had great potential. If you play the wrong game on clay and play well, you can still lose. You have to play smart as well. It's something I had to get really used to, especially when I was coming up against the best player like Rafa."
https://www.independent.co.uk/sport...a-volley-or-a-serve-its-too-easy-1979825.html