Open up players.atp or players.wta in the data folder and you can change character stats there. But keep in mind the skill normalizing will still take effect unless you turn it off in the tennis.ini file. You don't experience injuries in career mode.
Thanks for the CPU Players info. I am just thinking about how the level of the game improves through time so a power hitter in the early 90's might be at 80% but by 2000 the hardest hitters might be at 95% to represent the growth over time.