Likely not even just playing tennis but anytime involving extreme concentration ... even eating a junk food fave.
I liked the first shot of the dude behind the racket. INTENSE. As are the rest, for sure.
It's all about the passion, isn't it. Even you might give a similar expression flossing out that tid-bit of food-debris annoyingly lodged between your bi-cuspids.
No tennis playing involved.