Contester and Opponent

Contester noun - Someone who contests an outcome (of a race or an election etc.).

Opponent and contester are semantically related. In some cases you can use "Opponent" instead a noun "Contester".

Opponent noun - One that takes a position opposite another in a competition or conflict.

Contester and opponent are semantically related. Sometimes you can use "Contester" instead a noun "Opponent".