It's as fair a measure as anything, and the OP didn't just ask which was more physically demanding, as vaguely interpreted as that phrase could validly be interpreted, but also "tougher". Also validly interpreted many ways, but toughness is a term we usually associate with the quality of enduring physical trauma inflicted on the body, including violence. This is what differentiates it from endurance itself, from stamina, or even from grit.
And, as I've already pointed out in this thread, tennis has a very low rate of injury, and involves virtually no trauma to the body beyond the gruel of the length of its matches. The pounding of the joints on the hard courts doesn't come close to the trauma the body endures from the same thing in basketball due to the verticality of basketball combined with the nature of gravity. Really, the most brutal thing endured in professional tennis that isn't in professional basketball is the sun.