Rubuta ci gaba don wannan sakin layi - A farkon aikinsa Nielsen ya horar da shi a Finland, wanda shine jagorancin nesa mai nisa a lokacin, kuma ya koya daga masu gudu na Finnish. Ya lashe lambar farko ta Danish a 5000 m a 1930. Nielsen ya karya tarihin duniya na mita 3000 a Stockholm a ranar 25 ga Yuli 1934; ya fuskanci Janusz Kusociński na Poland, wanda ya rike rikodin da ya gabata na 8:18.8, kuma ya ci shi. Lokaci na nasara na Nielsen ya kasance 8:18.3; a matsayin rikodin duniya, an tabbatar da shi a matsayin 8:18.4, tun da dokokin IAAF sun buƙaci lokutan nesa da za a zagaye zuwa na biyar na na biyu. Daga baya a wannan shekarar, Nielsen ya lashe tagulla a cikin mita 10,000 a gasar zakarun Turai ta farko
a Turin, ya yi rashin nasara ne kawai ga Ilmari Salminen na Finland da Arvo Askola; shi ne Danish kawai wanda ya lashe lambar yabo a cikin taron.