Tennis as a sports category?

As far as I can tell, tennis is a unique sport. It is the only one that is played out of doors, on a court, with only 2 players who play directly against each other (in golf, they play one after the other). I know tennis has doubles games but basically it is played one against one -- it is not a team sport, such as soccer, baseball, cricket, basketball or even volleyball. Badminton and ping-pong are 1-on-1 but they are played indoors. Chess is a board-game, not a sport. Can anyone give me examples of other sports similar to tennis? Or is tennis in a cl by itself?