What is Tennis attire?

Anything they like.

Tennis spectators usually wear regular clothing as they normally would. If you are watching a professional game, just wearing regular solid shirts and shorts (maybe a polo shirt if you really wanted to) will be fine, but tennis spectators do not usually wear plaid shorts or anything like that. If you are watching a college or lower tennis match, pretty much anything would do, but no 'Volcom' crazy designed shirts or other brand name designed shirts. Just a solid shirt (polo shirt if you want) and solid shorts, or minimally designed shorts.