Red Star Belgrade will meet Vojvodina in next month's final after they beat city rivals OFK 1-0 with a late goal in the other semi-final on Wednesday.
Serbian first division top scorer Mrdja, who scored twice in his country's 3-0 World Cup warm-up win against fellow finalists Japan last week, continued his excellent run of form with a pair of headers from crosses by midfielder Dusan Tadic.
Winger Mario Djurovski sealed Vojvodina's passage to the final on the hour, rifling an unstoppable shot from 25 metres into the goalkeeper's bottom right corner.
The game was played under tight security following a serious incident during Wednesday's semi-final, when a fan was taken to hospital in critical condition from a gunshot wound sustained during the match.
