"Tennis is tiring."
I've only ever used つかれる as つかれた so that's good to remember, that it's actually a verb that can used like this
つかれる (疲れる) is a finite form of the verb. Did you mean to say a different word than "infinitive"? Usually つかれ (疲れ) would be the form of the verb which might get called "infinitive". ^^
no. 疲れている=疲れた describe the state of tiredness. 疲れる is to make someone tired
Tsukareru means to get tired. Literal translation is something like "when it comes to tennis, I get tired".