Willemsen was known for his powerful use of the Dutch language and his flawless style.
After completing his secondary education in Amsterdam, Willemsen enrolled at the Conservatorium to study the piano.
He decided, however, that literature was his first choice and, at a relatively late age, he began studying Portuguese.
van der Hoogt Award [nl] for Braziliaanse brieven.
Till his death on 29 November 2007, Willemsen was working on translating Pessoa's entire oeuvre.