Roger Federer's decision to withdraw from the French Open was "very tough," according to his co-coach.
The 18-time Grand Slam champion announced on Monday that he was withdrawing to concentrate on the second half of the season.
Federer trained on red clay for a few days before the announcement, Severin Luthi told The New York Times.
"We just wanted to have all the information," he told the newspaper, "to see how practice went and how he feels.”
Since Federer has not played a clay-court event this season, he would have had to adjust to and train on the surface. Luthi added that Federer's knee, on which he had surgery in 2016, was not a factor.
“You don’t have the guarantee that you are not getting hurt,” the coach said, “but really the knee is in perfect shape, so that luckily was not an issue.”
Federer won the first major of the season at the Australian Open.
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