Translation:Tennis is tiring.
In my opinion tennis is tiring means playing tennis is tiring. Same as テニスはつかれます vs テニスをするのがつかれます. These four all mean the same thing.
I thought Tennis is tiring, could also mean watching tennis is tiring. Or even hearing about it.
Or is that not possible.
Technically I at least get tired from it.
Yes, technically it is possible, but if we want to say something which is contrary to the common sense of the majority, we have to say it explicitly. Just say "tennis is tiring" is ambiguous so people apply the most probable one "playing tennis" instead of listening or watching.
This is called "こんにゃく文" (Konjac sentence)
「こんにゃくは太らない」(Literal translation: Konjac does not get fat.)
Literal translation does not work because there are some words omitted in the Japanese sentence.
「こんにゃくは私が食べても太らない食べ物だ」 "Eating Konjac does not make me get fat."
Similarly, テニスはつかれます is short for テニスは私がやって疲れる運動です Playing tennis makes me get tired.
Does the verbs here are modifying the nouns like 折れた剣? (I fail to understand that sentences btw)
Hi, yes exactly that the verb is describing the noun (like the which clause in English). No worries if you can't understand. This will probably be taught somewhere in Japanese tree 2.0. The point I was making is that this is a type of omission of parts of a sentence, just like omitting わたしは in most of the sentences you have come across.