Fiedler began his career in engineering at Stoewer and moved to Horch in 1924, where he eventually became chief engineer. He designed 8-cylinder and 12-cylinder cars at Horch. He left in 1932 when Horch became part of Auto Union. He then went to BMW on the recommendation of his former assistant Rudolf Schleicher, who had joined Horch from BMW in 1927 and returned to BMW in 1931. Fiedler became chief engineer at the age of 32 over former chief engineer Alfred Böning who became head of the automotive drawing office.