This is the 15899th most frequent Polish word.
tenisisty
"Tenisisty" means "of the tennis player" (possessive form) in English.
The word 'tenisisty' is used in the genitive case to indicate possession, describing the brother of the tennis player.
Ten brat tenisisty jest bardzo utalentowany.
This tennis player's brother is very talented.
Here, 'tenisisty' is in the accusative case and used with the verb 'have not seen', referring directly to the tennis player as the object of the verb.
Nie widziałem wcześniej tego tenisisty.
I have not seen this tennis player before.
In this sentence, 'tenisisty' occurs in the genitive case indicating that the opinions belong to the tennis player.