Every Saturday here in Orlando, the guys and I go play hard-court tennis outside, with temperatures above 90, humidity about the same, and with court temperature from the hard-court probably above 120-degrees. We play hard and run hard for three hours.
I sweat, I go through about 64 oz. of water, but I have a blast.
I guess one just gets used to it.
That's a fallacy. One's tolerance to heat and cold are genetic more than conditioned. That's a common misunderstanding of the variability (diversity) and adaptability of people. Some people are cold tolerant more than others and some people are more heat tolerant than others. To assume that we are all born the same and that one can condition or get used to those thresholds is one of the great health dangers. That's why there are different ideal climates for people and these sensitivities are not just all in their head; the body is giving them valuable information about its operating conditions. They shouldn't be running marathons on the hottest day of the century -- or playing tennis on torturous conditions. They should be playing them under optimal conditions for peak performances. Otherwise, they'd run the pole vault in a hurricane.