Stress
There are some of the usual stresses. Worry about money: Yeah, even as a professional athlete, income can be a concern. Worry about remaining competitive: No one's going to bring the axe down on your head, per se, but if you can't keep pace with your co-workers, it's going to be tough to make a living doing what you love. Worry about missing your family: Well yeah, if you're on the road all the time, you're going to be that absent father or mother that no one wants to be.
And yes, there are some stresses particular to tennis or other physical activities: namely, worries about unforeseen injuries. You know you could be in trouble when there's even a thing called "tennis elbow."
But all in all, you're playing a game, for goodness sakes. You get to travel the world, seeing new places, meeting new people, and then hitting a ball around for a few hours. It doesn't get much sweeter, or less stressful, than that.