What a bunch of wussies.
College football is going thru 2-a-days and these ATP pros cannot manage 2 or 3 sets of good tennis.
My son (17) had a 90 minute workout and then played a set today and its hotter here than in Cincy.
This is why tennis is still regarded as an elitist sport.
I'm choosing to leave your son out of this (everyone's kid practices for longer in hotter weather and harder than federer and and and).
If you play, you should at least be able to recognise that tennis is a brutal endurance sport at the high level? Why? Because it combines aerobic endurance with skill and a power/sprinting requirement, and let's not forget it is a full-body sport as well, requiring agility, endurance and power from the upper and lower body.
Many sports require one or two of the above elements, few require all of them, and very few team sports are as impacting on the body as professional tennis.
Maybe you haven't been to a pro tournament lately? I'm a tall male, reasonably fit and strong, I'm 31, and I hit the ball hard. Standing on the sidelines of the Sydney International practice courts this year for the first time in a couple of years, I was completely blown away by the levels pro tennis is reaching.... I remember watching Matthieu's forehand practice session - it was simply mind-blowing, and he was hitting them non-stop for 15-30 minutes in the heat, just to give one example. Pro tennis is a completely different game, and I have true respect for the athletic abilities of anyone playing it or trying to play it, especially those playing in a Master's tourney or GS in 40 degrees plus!