Learn English – Single word for quiz competition participant

You probably can't get closer than quizzer.

Curiously there is no definition in any dictionary and most only state it being a derivation from quiz, but ODO has several example sentences. From these sentences it seems quite fitting to your needs.

‘The quizmaster went back a happy man despite the turnout, as this was ‘a focused bunch of quizzers, who answered most queries’.’

‘Needless to say, young quizzers are encouraged.’

‘Nobody likes a clever-clogs, however, especially not on television and so many of the quizzers play strategically dumb during the audition process in order to increase the sense of dramatic tension.’

‘She sits back in her garden chair, turning away from the quizzer.’ - ODO