Looking on the internet deeply has found these results:

She is good at tennis, because he is not. is the most popular phrase on the web.

She is good in tennis, because he is not. 11 results on the web	She is good at tennis, because he is not. 13 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
Dude, you had to play me in tennis, now horse. Let's hope that baby likes it too, because he or she is most certainly not far off. That refugee should be a concern for countries a long way away from Poland, because he or she is likely to relocate. But in tennis, they're forced to interact. No, because he's not. Which is good, because he'll definitely be happier than if he lost. Lettered in tennis, squash, and served as editor of the school newspaper. She is all that is good in the world.	It doesn't look like this phrase is very popular! The search could not find any examples on the internet.

She is good in tennis, because he is not.

11 results on the web

She is good at tennis, because he is not.

13 results on the web

More popular!

Dude, you had to play me in tennis, now horse.
Let's hope that baby likes it too, because he or she is most certainly not far off.
That refugee should be a concern for countries a long way away from Poland, because he or she is likely to relocate.
But in tennis, they're forced to interact.
No, because he's not.
Which is good, because he'll definitely be happier than if he lost.
Lettered in tennis, squash, and served as editor of the school newspaper.
She is all that is good in the world.

It doesn't look like this phrase is very popular! The search could not find any examples on the internet.