I play tennis 3+ times a week by playing in tennis clinics. So I'm doing drills and also playing a lot of doubles points. In the clinics the instructors say I'm like a machine, since I don't ever show sign of fatigue. However, when I play a competitive match it's another story completely. When I'm playing all out in a set I can feel pretty fatigued after playing long points within the 1st 3 games. As the set goes on I try to not show my opponent, but I feel exhausted. Next week I'm playing someone in a tournament where I know I can beat him as far as the tennis points, but it requires hitting a lot of balls, a lot of running side to side, and then quickly closing out the point at the net. Any ideas on how I should train? Why do I become exhausted playing sets but not playing in clinics? Anything I can do during the match to stop tiring?