The opposite of tennis

Today we were talking about opposites.

Rosie: "The opposite of sitting down is standing up."
Holly: "The opposite of dark is light."
Rosie: "The opposite of big is little."
Holly: "The opposite of clean is dirty."
Rosie: "Daddy... what's the opposite of tennis?"
Daddy: "Well... some things don't really have an opposite."
...
Holly: "I have an opposite. It is the opposite of tennis."
Daddy: "Wow. Go on then, what's the opposite of tennis?"
Holly: "Not-Tennis."