This has happened twice before but not as severe as today. I'm a 4.0 player and I was practicing groundstokes for 2 hrs. today with one of the local college players. I was hitting great strokes for my level and decided to serve some at the end of the day. My first serve was good but "every" serve I tried after that I was hitting the throat of the racquet or the frame. My back did feel a little sore from hitting so hard but it didn't seem to effect my groundstrokes. I must have tried 50 balls and with every serve this was happening. I had to quit because the rest of the college team was arriving and it was pure embarrasement because I could not figure out what was wrong. The serve stroke felt fine but there was obviously something serious going on... could it have been just being tired. Please help!