Originally Posted by VeeSe
Better fitness is your answer. If you have no problem with balance and court orientation after 5 balls, there's no reason why the others should be different unless your body is getting too tired, then other things start to come into play. Cardio training could help a lot (some long distance running, interval workouts like 400m repeats etc can go a long way).
I was going to say this. Yes, it appears it's a physical conditioning issue. When you get tired your technique suffers and when your technique suffers your mind suffers and when your mind suffers your result suffers. It happens to all of us even to the top players.