Abstract: A racket sport training device is disclosed. The training device is usable in conjunction with any strung racket, and includes a pocket-like structure fashioned from a flexible, rip-stop nylon material. The opening of the pocket has a perimeter that approximates the perimeter of the racket frame to which it is attached, and, when the racket is used with the training device in place, air is entrapped in the pocket. The entrapped air adds additional stress to the swing so that the player using the device for warmup exercise faces higher, racket-head resistance. This results in faster warmup with the encumbered swing constituting a functional exercise--one that uses the same musculature as is used in actual play. The device is further optionally adapted for tennis service training by having one or more zippers to bleed air from the device. A scale marking is provided adjacent the zipper for reproducibly setting the level of the racket head resistance desired for service warmup.