It's rather complicated, I've figured out a good part of it here: http://orden-y-concierto.blogspot.com/2013/04/wii-sports-tennis-skill-points-system.html
Edit: In a nutshell: your score during a game determines a certain amount of skill points to tend to, which ranges from 0 to 1200, according to a table. Your rivals' skill applies an offset to that amount, that can be as high as 1200 when playing against the hardest rivals. If you scored always the same score against the same rivals, your skill points would tend asymptotically towards a certain value. In particular, you can't get 2400 points, because 2400 is the asymptote when winning 40-0 against the hardest possible rivals. If you were able to lose always 0-40 against Sarah/Elisa without changing rivals, your score would tend asymptotically to 1200.
Edit 2: There is a simple way to figure out the base asymptote (before being offset by the rivals' skill). If you lose, it is 10 times the points you got (that is, 0, 150, 300, 400). If you win, it is 1200 minus 10 times the points the rivals got (that is, 1200-0=1200, 1200-150=1050, 1200-300=900, 1200-400=800).
What I haven't been able to figure out yet is how the next rivals' skill is decided as games progress.