I just started taking some lessons with a good coach. He's had some great observations and advice so far, but I'm struggling with something he suggested on serve. He told me to watch the ball from the beginning of the motion. Normally I start looking at the ball around shoulder height. I really struggled to watch the ball throughout. It throws off my balance. So I looked at some videos and found that Sampras, Federer, Agassi, and Becker all make eye contact with the ball around shoulder height. The one benefit I've found so far to watching the ball throughout is that it forces me to move more slowly, which improves racquet acceleration before impact. I intend to ask him later if he intends me to watch the ball all the time or just for right now while I slow down my rhythm. What do you guys think about this? When should you start looking at the ball on serve and why?