Similarly for men, the pockets inside elite tennis players shorts are extremely thin, almost translucent, to ensure they are as streamlined and aerodynamically efficient as possible, and a ball with an "itchy nap" can become a significant, possibly point, game, set or even match loosing factor...
The ferosity of the nap on tennis balls varies ball to ball and so women like to inspect the immediately available balls fairly carefully to avoid sticking a ball with what is technically termed "an itchy nap" inside their panties.