tennis player vs participant

tennis player and participant both are nouns.

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

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

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

tennis player (noun)	participant (noun)
an athlete who plays tennis	someone who takes part in an activity
	a person who participates in or is skilled at some game