Club Nielsen began his youth football by the local club
Hjørring IF and with them his professional career in 1966, before moving to
KB in 1970. In 1971, he signed for Switzerland's
Winterthur and he played there under head coach
Willy Sommer until 1974. He soon became an strategist within the team with his brilliant ideas. However, despite reaching the
Swiss League Cup final twice, he was still denied success in the form of a title. Due to financial reasons Winterthur sold Nielsen to
FC Basel during the summer break. Nielsen joined Basel's first team for their
1974–75 season under head coach
Helmut Benthaus. He played his debut for his new club in the
Cup of the Alps, an away game in
Nîmes, on 20 July 1974. He not only scored his first goal for his new team in same game but Basel won 4–2 thanks to a
hat-trick from Nielsen and a goal from
Ottmar Hitzfeld. After playing in two further games in the pre-season Cup of the Alps and two games in the
Swiss League Cup, in which he scored two further goals, Nielsen played his
domestic league debut for the club in the home game in the
St. Jakob Stadium on 17 August as Basel played a 2–2 with
Xamax. He scored his first league goal for his new team in the away game in the
Stadion Wankdorf on 12 October. In fact he scored twice as Basel played a 4–4 draw with the
Young Boys. At the end of his first season with the club, Nielsen celebrated a Basel win in the
Swiss Cup final as they defeated
Winterthur to win the trophy. Then at the end of the
1976–77 Nationalliga A season Basel won the Swiss Championship, winning the play-off match against
Servette 2–1, after the two teams had ended the regular season level on points. Nielsen stayed with Basel for four seasons. Between the years 1974 und 1978 Nielsen played a total of 164 games for FC Basel scoring a total of 30 goals. 99 of these games and 18 goals were in the Swiss domestic league. 21 games were in the
Swiss Cup or
Swiss League Cup, 23 were international cup games and 30 were test games. Nielsen scored his only European goal on 29 September 1976 in the second leg of the first round of the
1976–77 UEFA Cup. Basel played at home in
St. Jakob Stadium and won 3–0 against
Glentoran F.C. In 1978 he signed for
Luzern who played in the second highest tier at that time and that season Luzern won promotion.
International Nielsen was
capped ten times by the
Denmark national team, making his international debut in Denmark's 3-2 win over
Japan on 28 July 1971 in
Parken,
Copenhagen. His last match for the national team was in a 6-1 defeat to
Romania on 23 August 1975 in
Bucharest. ==Private life==