Career
Mertelj started his career playing for Triglav Kranj and stayed there until he was acquired by Slovenian top division club, Koper. He played for Koper during the 2008-2009 season, making 35 league appearances for the club and scoring three goals in the process. He then transferred to and is part of the thirteen times Slovenian champions ever since.
Mertelj made his international debut on 26 May 2012 on a friendly match between Slovenia and Greece, coming in as a late substitute.