signed a contract with Nordsjælland on 25 September, and made his debut in October. He played 10 games in his first season with Nordsjælland, as he was backup for Martin Bernburg. As Bernburg was sold, Bille got his breakthrough in the 2009–10 Danish Superliga season, when he played 32 of 33 games and scored eight goals. He was also a part of the Nordsjælland team which won the 2010 Danish Cup. Nicki Bille gained his first team debut 5 February 2011 in a home fixture against Levante FC, Villarreal CF lost the game 0–1. He was loaned out to Rayo Vallecano for the 2012–2013 La Liga season. On 3 January 2013 he signed a four-year deal with Norwegian side Rosenborg after ending his stay at Rayo Vallecano earlier than planned. Billie Nielsen changed from number 9 to his favorite number 14 before the 2014 season. In 2014, Rosenborg sold Nielsen for an undisclosed amount to the French club Evian Thonon Gaillard F.C. During his Rosenborg career, he had scored 24 goals in 50 games. On 1 July 2015, he signed a three-year contract with Esbjerg fB, but his time at Esbjerg was never a success. He left half a year after signing with the club having played 19 games and scored five goals. On 27 January 2016, he signed a 2.5-year contract with an option of one additional year with Polish champion Lech Poznań. He was given shirt number 19 after the former topscorer Kasper Hämäläinen. Bille got an injury in February 2016 in a match against Podbeskidzie Bielsko-Biała, where he only played 25 minutes before leaving the pitch. He was out for 1 month . In August 2016, Bille suffered an injury in his groin that kept him out for the next seven months. On 30 January 2018, Panionios F.C. officially announced the purchase of experienced Danish international striker Nicki Niels Bille Nielsen from Lech Poznań, for an undisclosed fee. In July 2018 he signed a contract with Lyngby Boldklub, who was managed by Bille's former youth coach Mark Strudal. On 26 October 2018 he was sacked from Lyngby following an episode, where he was arrested and charged by the Copenhagen Police Department. On 25 July 2019, Bille joined Ishøj IF in the Denmark Series – the 4th tier of Danish football. He was released without making an appearance, on 20 November 2019. In June 2020 he joined VB