This is the 17407th most frequent Russian word.
теннису
"Теннису" means "to tennis" (dative case of "теннис").
In this sentence, 'теннису' is in the dative case, used to express indirect objects.
Я всегда восхищался теннису.
I have always admired tennis.
Here, 'теннису' is in the dative case, indicating the entity to which the action of dedicating is directed.
Он посвятил всю свою жизнь теннису.
He dedicated his entire life to tennis.
In this sentence, 'теннису' is in the dative case used after the preposition 'к,' which shows a directional or topical context.