born in fribourg, grether started his youth football with local team fc reinach. in 2006, he transferred to basel and there he progressed through the ranks. he played in their u-16 and u-18 teams. with both he won the swiss championship. later he played in their u-21 team before he signed a two-year professional contract on 24 may 2012. he then joined their first team. he made his first team debut in the swiss super league on 13 july 2012, being substituted in, during the 1–0 away win against servette. he played his first european match in a uefa champions league preliminary stage match against flora tallinn on 24 july 2012 as basel won 3–0.