I am 36. During the summer season the most I can play is 5days/week, with 2-3 days playing sets. But I always feel if I play sets on 3rd day in a row, I wouldn't have the same level of focus as on the 1st day. This leads me to wonder how the pros maintain their mental focus on top of maintaining the physical condition. It's amazing since they are playing for a living and I am sure every game, every point means a lot more than for us recreational players. Maybe it's just an age thing... I can certainly remember myself playing with better focus when I was 26.