In 1927, Müller turned professional, making his pro debut at light heavyweight in April that year. He was unbeaten in close to twenty fights by the end of 1928, including wins over former Scandinavian middleweight champion Martin Tancred, future French champion Abel Argote, former German (VDF) heavyweight champion Hans Breitenstraeter, the only fight he failed to win during this period a controversial draw with future British champion Harry Crossley which resulted in the referee being suspended by the German authorities and Crossley instructed to omit the fight from his record.
