You make use of the common gauge (the one that can rise up to 4 times) by using either L+R+A or L+R+B for common shots and serves, or L+R+Select for some passive skills which will help you. Please note you are allowed to use one of these only when your character is blinking white.
You can also use L+R+Start to use the golden "Awakening" bar (filled slowly during the game or using L+Select taunt) to have unlimited use of the specials until the Awakening bar ends
ok this is like the funnest game ever, its my favorite game so far.. i like these way better than long tedious rpgs and side scrollers. and it helps me practice for when i really play tennis haha