professional tennis player vs tennis pro

professional tennis player and tennis pro both are nouns.

Word	Noun	Adjective	Verb	Adverb
professional tennis player	Yes	No	No	No
tennis pro	Yes	No	No	No

As nouns, professional tennis player and tennis pro are synonyms defined as:

professional tennis player and tennis pro: someone who earns a living playing or teaching tennis

professional tennis player (noun)	tennis pro (noun)
someone who earns a living playing or teaching tennis	someone who earns a living playing or teaching tennis