It's been a long time since I last played that trash. But, in the RPG mode, when you leave the village you end up on a world map screen and, as you blunder about, you're occasionally challenged to a game of tennis, just as you'd run into random encounters in a regular RPG. If you win, you earn money that you can use to purchase new equipment in the towns. You need to explore the map a little and find the courts where the "local champs" hang out. Beat the champ, move on to the next town, repeat the process, much like in the far-superior Final Lap Twin.
You might have to solve some minor-league "puzzles" or search for some secret items along the way. I've never made it more than two or three towns into WCT's RPG mode because the game blows.
As far as normal mode goes, don't the players have "grades" in different areas that tell you how good they are at certain things? I can't remember if the grades are in the manual or actually displayed during the game.