protivnica

1. (in sports, games) opponent; rival
   Naša teniserka je lako pobedila protivnicu u dva seta. — Our tennis player easily beat her opponent in two sets.
2. (+ gen.) opponent
   Ona je oduvek bila protivnica te reforme. — She has always been an opponent of that reform.
3. adversary; opponent
   Na izborima se suočila sa starom političkom protivnicom. — In the election she faced an old political adversary.

In the sense of opposing something it takes the genitive: protivnica rata.