There are certain speed standards in each age category and at each performance level. You can have the best attitude, you can be mentally strong, motivated and focused during game at 100 percent, but if you are out of these standards you have no chance in a match. For example, if at ATP level you can't serve around 200 km/h on a regular basis or you are not able to return to the court serves at 210 km/h, your motivation and mental part of the game will hardly help you at this time.
Power without control in tennis is useless. If you have this problem, you will need generate more spin in your shots, more about spin generation here.
If you want play like a pro we recommend you our Analysis of All Strokes.