Synonyms for Opponent (same or very similar meaning)
WordNet sense 1 (a contestant that you are matched against):
opposite, oppositionWordNet sense 2 (someone who offers opposition):
adversary, antagonist, resister, opposerWordNet sense 3 (characterized by active hostility):
opposingWordNet sense 4 (characterized by enmity or ill will):
hostileFrom the ODE community, based on WordNetadd/edit