Rafael Nadal struggles indoors because he cannot get as much out of the bounce, says Serena Williams' coach Patrick Mouratoglou.
For all Nadal's incredible success, only one of his 86 career titles has come on an indoor hardcourt, and that one came more than 15 years ago.
70% of his titles have come on clay, and a large reason for that has been how much extra bounce he can get out of the surface with his top spin.
By forcing opponents to play the ball higher than shoulder height, Nadal can strip them of their power and dictate points.
However, according to Mouratoglou, that's just not possible indoors and it goes a long way to explaining why he struggles to win tournaments in those conditions.
“What makes Rafael Nadal so different and so difficult to beat on other surfaces, he doesn’t have that advantage indoors on hard courts because the bounce is low," explained Mouratoglou.
"There is a reason why on hardcourts indoors, the bounce is lower than on hardcourts outdoors.
"The surface is named hardcourts in both cases, but it’s not the same surface at all."
Nadal is battling to win his first ever ATP Finals title this week, and he has made a fine start beating Andrey Rublev in straight sets in his opening match.
For more exclusive content from Tennisbuzz, including news, features, trivia, promotions and more, please be sure to sign up for our newsletter below or via our home page.
Why not test your tennis knowledge by tackling one of our fiendish quizzes too?