tennis player vs athlete

tennis player and athlete both are nouns.

Word	Noun	Adjective	Verb	Adverb
tennis player	Yes	No	No	No
athlete	Yes	No	No	No

As nouns, athlete is a hypernym of tennis player; that is, athlete is a word with a broader meaning than tennis player:

Other hypernyms of tennis player include jock, participant, player.

tennis player (noun)	athlete (noun)
an athlete who plays tennis	a person trained to compete in sports