tennis pro vs professional tennis player

tennis pro and professional tennis player both are nouns.

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

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

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

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