Rafael Nadal ’s triumph at this year’s United States Open reinforced the argument that he might be the greatest men’s tennis player ever — outdoors.

It’s a small but notable distinction on the eve of the Paris Masters and the ATP Finals . Nadal loves Paris in spring, winning a dozen French Opens, yet he has never won the indoor Masters 1000 tournament just across town. Nor has he ever captured an ATP Finals title, which is also played indoors.

This is partly because Nadal grinds so hard that his body wears down by year’s end. Last year he withdrew from the Paris Masters and the ATP Finals because of an abdominal injury; the year before, a knee injury ended his season in the Paris quarterfinals; and in 2016, he was out with wrist woes.

But indoor tennis is different enough to negate some of Nadal’s brilliance while allowing others to thrive. It’s an environment that helps servers pinpoint location and be more aggressive, and it favors players who like to attack early or counterpunchers who hit flat.