During
Operation Barbarossa, Mertens shot down DB-3 bomber on 26 June 1941, and then during July 1941 he shot down eleven more Soviet aircraft including three in a day on 16 July 1941. He was awarded the Iron Cross 1st Class. On 23 July 1941,
Oberleutnant Viktor Bauer, the
Staffelkapitän (squadron commander) of 9.
Staffel, was injured in a landing accident. In consequence, Mertens was appointed acting commander, a position he held until 6 November. Mertens shot down seven more aircraft in August 1941, one more in September, and another in October 1941. In February 1942, Mertens transferred to the Staff of
Jagdgeschwader 3 and was promoted
Oberleutnant before joining the staff of III.
Gruppe of
Jagdgeschwader 3. He was awarded the
German Cross in gold on 27 May 1942, or 8 June 1942. Between 6 April 1942, and 26 July 1942, he shot down fifteen more Russian aircraft, primarily
Lavochkin-Gorbunov-Gudkov LaGG-3 and
Polikarpov I-153 fighters and
Ilyushin DB-3 bombers, bringing his tally to forty confirmed victories. In late July 1942, Mertens was appointed to command the 1.
Staffel (squadron) of
Jagdgeschwader 3 and promptly shot down seven more Russian fighters and bombers during the next four weeks. A photograph in
Luftwaffe Rudder Markings 1936-45 taken on 22 July 1942, in the
Stalingrad sector shows him posing with the tail of his
Bf 109 which is marked with forty-seven victory tabs. He was awarded the
Knight’s Cross of the Iron Cross on 4 September 1942, the recommendation at the time stated that he had achieved fifty victories in air combat. On 2 October, he was temporarily appointed
Gruppenkommandeur (group commander) of I.
Gruppe of JG 3, replacing
Hauptmann Klaus Quaet-Faslem until 2 November 1942. Documentation concerning Mertens service in the next eight or nine months from the time of his award until July 1943 is scarce and no details of his actions as a fighter pilot are known. ==Western Front==