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.[2][3] At the 1994 Australian Open, Nielsen and partner Jean-Noel Grinda Jr made the boys' doubles semi-finals.[2] The following year he and countryman Teo Susnjak won doubles titles at the Australian Hardcourt Junior Championships and Philippines Junior Championships.[2]
In 1997, Nielsen began playing Davis Cup tennis for New Zealand.[4] He took part in a World Group play-off tie against Spain in 1999, which the New Zealanders lost without winning a match.[4] Nielsen lost both of his singles rubbers, to Félix Mantilla and Francisco Clavet, but did take the former into a fifth set.[4]
Nielsen was a wildcard entrant at the 1998 Heineken Open and in front of his home crowd defeated Brazilian Fernando Meligeni in the opening round, before being eliminated by Marcelo Ríos.[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] With partner James Shortall, Nielsen also made the quarter-finals of the doubles that year.[5] He was a doubles quarter-finalist again at the Heineken Open in 2003 and 2004.[5]
At Islamabad in 2004, Nielsen played the longest match in New Zealand's Davis Cup history, against Pakistan's Aisam Qureshi. The five set match lasted for five hours and 20 minutes.[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".[7] He was given a two-year suspension by the International Tennis Federation.[7]
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.[4] His 20 wins for New Zealand in singles also has him second all-time, to Onny Parun.[4]
New Zealand
New Zealand
