Opponent vs. Adversary vs. Antagonist

Adversary,Antagonist and Opponent mean a person or a group expresses or manifests an opposite position.

opponent implies little more than position on the other side,as in a debate,election,contest ,or conflict.
- Norris twice knocked down his opponent in the early rounds of the fight...
- His bad behavior provided plenty of ammunition for his opponents.
- the president put the blame squarely on his opponent.
- In debate he was a formidable opponent.
- Rodgers was not an opponent of the new airport.

Adversary is used formally or in literature and implies sharper ,often more personal ,opposition in a struggle for supremacy; an enemy who fights determinedly continuously and relentlessly; a
or someone or a country that has strongly different aims and beliefs from yours. 

- Symon grabbed his adversary by the throat and wrestled him to the ground.
- His political adversaries are creating a certain amount of troubles for him.
- He saw her as his main adversary within the company.
- His sense of humor allowed him to face adversary with equanimity.
 
Antagonist suggests one who in hostile spirits opposes another ,often in a particular contest or struggle,Your antagonist is your opponent or enemy.

- Spassky had never previously lost to his antagonist.
- A duel with an antagonist.
- The woman was forcing her antagonist's face into the mud.
- Allegations concerning his arch political antagonist.