Sielaff initially joined Audi in 1984 as a student intern. He was awarded an Audi scholarship and went to London to study vehicle design at the Royal College of Art (RCA) where he was awarded the MDes(RCA) qualification. While at the RCA he won the Triplex prize for his ACID (Advanced Concept: Induction Drive) three-wheeler project which bridged the gap between motorcycle and sports car and included innovative glazing.
Immediately upon completing his course in 1990 he returned to Audi where he worked on interiors at Audi's Ingolstadt HQ before moving to Audi and Volkswagen interiors in Munich. In 1995 he moved to Sitges, Spain before returning to Munich in 1997 to run the Audi Design Centre and then quickly moved to Ingolstadt to become head of interior design of Audi.
In 2003 Sielaff moved to rival Mercedes-Benz to become the design director of Interior Competence Center in Sindelfingen. He returned to Audi in 2006 to be Head of Audi Design under Walter de Silva to replace retiring head Gerhard Pfefferle.