As part of Emoji, there are a number of sport ball encoded as characters, and some awkward stuff like "TENNIS RACQUET AND BALL" (U+1F3BE). Although the later is displayed in some places as an standalone tennis ball symbol, I believe that an actual "tennis ball" character may be good. I'm not very fan of sports, but I have seen it used a lot in real-time notation of serves in scoreboards, particularly on TV and websites. In more primitive stuff is replaced by a big asterisk (which I'm not sure which is the proper character to use either, but that's another issue). Not sure if it applies to other sports, but is the only one I remember such a symbol used.
