single combat vs fighting

single combat and fighting both are nouns.

Word	Noun	Adjective	Verb	Adverb
single combat	Yes	No	No	No
fighting	Yes	Yes	No	No

As nouns, fighting is a hypernym of single combat; that is, fighting is a word with a broader meaning than single combat:

Other hypernyms of single combat include combat, fight, scrap.

single combat (noun)	fighting (noun)
a fight between two people	the act of fighting; any contest or struggle

single combat (adjective)	fighting (adjective)
	engaged in or ready for military or naval operations