While Mario Tennis is one of Nintendo's best known Mario spin-offs, I missed the one for Game Boy Color since it came out in 2001. It had received a lot of critical acclaim back then, and some said it was even better than the N64 Mario Tennis.
One of my biggest fears about Mario Tennis from Game Boy Color was the system limitations. The GBC is about the same power as the NES, and Tennis on NES is a very plain sports game that can't be enjoyed for terribly long. However Mario Tennis on GBC plays very fast and it can be just as fun as its 3D predecessor and sequels.
The game also features a story-mode where you pick a character not from the Mario universe and train them in a RPG-ish tennis academy. You can level up your skills as you face tougher tennis opponents. You also get a doubles partner that can also level-up.
Overall while Mario Tennis plays better than I expected, I really didn't like it as much as other Mario Tennis games. While I could see the story mode being a big deal in 2001, in 2014 I found it more tedious than fun. I also hated playing as a normal person than one of my favorite Mario characters. It's a decent game, but it hasn't aged that well in my opinion.
Score: 7 out of 10