Are you a tennis fan or do you know someone who ardently follows every tennis tournament that happens? If your reply is yes to either one of these questions or both, then probably what you could go out and get is a tennis ball keychain. This keychain is available in various sizes, though not the actual tennis ball size. Though you could convert an actual tennis ball into a keychain by attaching the ring and small chain.
You could probably find a tennis ball keychain autographed by your favorite tennis player at a store dealing in sports memorabilia. And just in case you do not get the one you are looking for then you could purchase a keychain in the shape of a tennis ball and get it autographed from the tennis player when you attend their tournament or get a chance to meet them on a one-to-one level.
Some people who attend international tournaments try to get a tennis ball that was used during the match and then convert it into a keychain. However, during important tournaments there are stalls inside and around the venue that sell such key chains.
You could gift someone a stress ball keychain that resembles the tennis ball. In fact they would be more than happy to receive such a gift if they are overworked and have been missing all that has been happening in their favorite sport. This can also be gifted to those who regularly indulge in this sport as a leisure activity.