Nielsen was Denmark's greatest male athlete during the 1950s and his ability to draw large crowds meant that he was much in demand by his country's promoters.[6] His hectic schedule eventually took its toll on his health, and, with his times strongly affected by illness and exhaustion, Nielsen announced in September 1956 that he would retire after the 1956 Olympics in Melbourne.[6] Though he won his 800 metres heat in a time of 1:51.2, Nielsen chose not to run the semi-final and instead chose to focus on the 1500 metres.[7] In what proved to be the final race of his career, Nielsen finished the 1500 metres final in tenth place, over four seconds behind the winner, Ronnie Delany.[8]
