Abstract: A bag for storing tennis balls has an hermetic enclosure formed of flexible material, an opening larger than a tennis ball into the enclosure, the opening having a hermetic sealing zipper by which the opening may be opened and closed, and a flexible tubing from outside the enclosure through the flexible material, the flexible tubing having an open end adapted to engage an air pump, and a one-way valve adapted to admit air into the enclosure and to prevent air from exiting the enclosure through the tubing. Tennis balls placed into the bag are placed under pressure higher than the pressure inside a tennis ball by closing the opening by closing the hermetic sealing zipper, engaging an air pump to the open end of the flexible tubing, and operating the air pump until a predetermined pressure is established in the bag.