Nielsen had a highest junior ranking of 16th and during his early career on the juniors circuit had wins over
Dominik Hrbatý,
Mark Philippoussis and
Paradorn Srichaphan. At the
1994 Australian Open, Nielsen and partner Jean-Noel Grinda Jr made the boys' doubles
semi-finals. He took part in a World Group play-off tie against
Spain in 1999, which the New Zealanders lost without winning a match. He competed in the Heineken Open a further seven times and also reached the second round
in 2002, with a win over seventh seed and world number 33
Andreas Vinciguerra. In 2006, a sample that Nielsen had provided while playing the qualifying stages at the
Australian Open, tested positive for
finasteride, which can be used as a masking agent. An independent Anti-Doping Tribunal accepted Nielsen's claim that he took the product containing the positive substance to treat hair-loss, but found that by failing to check whether the hair-loss product contained such a substance indicated a "serious dereliction of duty on the part of any player who participates in a sport governed by the WADA Code". He was given a two-year suspension by the
International Tennis Federation. The ban effectively ended his career, but once the suspension ended he did briefly return to compete on the
Futures circuit and he played a Davis Cup tie in
Kuwait. That would be his 20th and final Davis Cup tie, a tally bettered only by
Onny Parun. He finished with 25 Davis Cup wins to his name, from 43 rubbers. His 20 wins for New Zealand in singles also has him second all-time, to Onny Parun. ==Challenger titles==