fielder vs ballplayer

fielder and ballplayer both are nouns.

Word	Noun	Adjective	Verb	Adverb
fielder	Yes	No	No	No
ballplayer	Yes	No	No	No

As nouns, ballplayer is a hypernym of fielder; that is, ballplayer is a word with a broader meaning than fielder:

fielder: a member of the baseball team that is in the field instead of at bat
ballplayer: an athlete who plays baseball

Other hypernyms of fielder include baseball player.

fielder (noun)	ballplayer (noun)
a member of the baseball team that is in the field instead of at bat	an athlete who plays baseball
a member of the cricket team that is fielding rather than batting