The Oxford English Dictionary (OED – 2nd edition, 1989) defines the phrase tennis, anyone?, also anyone for tennis?, who’s for tennis?, etc., as follows:
a typical entrance or exit line given to a young man in a superficial drawing-room comedy, used attributively of (someone or something reminiscent of) this kind of comedy. Also in extended uses.
I was too lazy to look for a Tennis topic past page 3. I'll merge it his if I come across one when I'm not being too lazy.