Kinect Sports Rivals

Continue to play Tennis until you have reached Level 10. From what I can tell, experience will only fill up your bar to the end. For example, if you need 5,000 points to reach Level 9 and you get 6,000 points, the bar will only fill up the 5,000. You will need to play another game of tennis to continue leveling up. To gain more experience, try more complex shots and don't be shy with your power-ups.

Update: as of early May a patch seems to have fixed the above issue. Lately when I finish a round, the bar will fill up with experience and then go to the next level if it completely fills up. Hopefully that will make these go faster.