[2][3] At the 1994 Australian Open, Nielsen and partner Jean-Noel Grinda Jr made the boys' doubles semi-finals.

[5] 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.

[5] At Islamabad in 2004, Nielsen played the longest match in New Zealand's Davis Cup history, against Pakistan's Aisam Qureshi.

[6] 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.

[7] 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".