I have the same problem. I played the mystery match against my rival 15 times today and i can't win no matter what I do.
I've won 2 slams and 5 masters already (15.1. in 3rd season) without any problems whatsoever. I've beaten pretty much everyone from the top 10 but I can't get even close to my rival. I struggle to get the first game, lose the second 0-40, struggle to win my service and lose almost always, lose her service 0-40 and lose my last service. I tried pretty much everything I learned in this game while playing other opponents, but noone has given me any real problems until now.
I even tried playing Federer with my girl in exibition, and won 3-0 3-1....i don't get it..what's the secret behind your rival?