With a singles game that featured consistency and excellent mobility, Nielsen played at a high international level from the late 1950s through the mid 1960s. In 1964 he won men's singles at the All England Open Badminton Championships, then considered the unofficial World Badminton Championships, narrowly defeating fellow Dane Henning Borch in the final. He played Thomas Cup (men's international team competition) singles for Denmark in the '60–'61 and <a href="1964 Thomas Cup">'63–'64