After a 10 year break, I started playing about 2 years ago. At first, I would be sucking wind in between points, and sometimes sore for days after a hard match. Now, playing the same guy or level of opponent, I am not that winded. If I played and hour and a half of tennis when I wasn't in shape against opponent x, and play opponent x now while I am in shape, am I burning the same amount of calories?