Frederik Nielsen

Nielsen made his ATP singles debut in Copenhagen, losing to Magnus Larsson in the first round d 6–1, 6–1. In 2003, he made his Davis Cup debut in the Europe/Africa Zone Group II tie against Tunisia at Hillerod, Denmark and defeated Malek Jaziri 6–1, 6–4 in the dead rubber, Denmark winning 4–1. Nielsen also won the singles title at the Futures event in Vietnam.

In 2004, Nielsen teamed with countryman Rasmus Nørby to win 5 Futures doubles titles. Reached 1 Futures Singles final. He won 1 doubles Futures title and 2 titles in 4 Futures singles finals in 2005. In 2006, he won doubles title at 2 Challengers and 7 Futures. Also won 3 Futures singles titles.

In 2007, Nielsen won doubles titles at 3 Challengers and Futures. 2-time singles winner at Futures. In 2008, Nielsen won 2 doubles Challengers and 1 Futures, all three with different partners. In 2009, he won 3 Challengers tournaments in doubles and 1 Futures title in singles.

In 2010, Nielsen won 2 Challengers and 3 Futures in Men's doubles. He also won singles Futures event in Italy. The following year, he won 5 Challengers doubles titles with 3 different partners; he was the finalist at 3 other events.

Nielsen represented Denmark at the 2012 Hopman Cup alongside Caroline Wozniacki, but the pair could not get past the Group Stages. Nielsen had a 1–4 Win-Loss record at the event, 1–1 in doubles and 0–3 in singles. He played much higher ranked opponents in Tomáš Berdych, Grigor Dimitrov and Mardy Fish. He managed to win 1 set against Fish but eventually lost 4–6, 7–6, 6–4. Nielsen and Wozniacki still managed to pull out the doubles match against Fish and Bethanie Mattek-Sands 7–5, 6-3 and won their only tie against USA.

Nielsen won the 2012 Wimbledon Championships doubles crown alongside Jonathan Marray.

He then played at the Australian Open and qualified for the main draw at a Grand Slam for the first time but lost to Kevin Anderson 6–1, 6–2, 6–4 in the first round. In February, Nielsen was selected for the Davis Cup Europe/Africa Zone Group I tie against Slovenia, which Denmark lost 5–0.

Nielsen had been friends with Jonny Marray since their early days on the senior tour,[2] but had only played together twice before in 2006[3] and 2008.[4]

Playing doubles with Marray in June, their first pairing of the year, they reached the Nottingham Challenger finals, losing out to Treat Huey and Dominic Inglot in three tight sets 4–6, 7-6(9), 8-10. They compiled a 10-6 match record in 6 tournaments.

Then Nielsen and Marray received a wild card at the All England Club and began their 2012 Wimbledon Championships with a 5-set win over Marcel Granollers and Marc Lopez, one of 4 teams they beat en route that qualified for Barclays ATP World Tour Finals. In the third-round, they edged out another five set victory this time against Aisam-Ul-Haq Qureshi and Jean-Julien Rojer 7–5 in 5th set, followed with victory over James Cerretani and Edouard Roger-Vasselin in the quarters in five sets again. In the Semi-Finals they caused the biggest upset by beating No. 2 seeds Bryans in 4 sets 6–4, 7-6(9), 6-7(4), 7-6(5). In the title clash they defeated the No. 5 seeds Robert Lindstedt and Horia Tecau in 5-sets 4–6, 6–4, 7-6(5), 6-7(5), 6–3. Marray was first British doubles champion at Wimbledon since 1936 and Nielsen was first Dane to win the doubles crown.

Following Wimbledon, they played together in 4 tournaments (2-4 record), losing first round in Winston-Salem, second round at Us Open losing to Jesse Levine and Marinko Matosevic having beaten the Italian duo of Flavio Cipolla and Fabio Fognini in the first round. They had a quarter-final in Basel losing out to Mariusz Fyrstenberg and Marcin Matkowski.

Nielsen and Marray made their debut at the Barclays ATP World Tour Finals in London, qualifying for the season finale as a result of their Wimbledon crown, and reached the Semi-Finals losing out to eventual champions Marcel Granollers and Marc Lopez 4–6, 3–6. In the Round-Robin stage the pair defeated top pairs including Indian pair of Mahesh Bhupathi and Rohan Bopanna 6–4, 6-7(1), 12–10, Max Mirnyi and Daniel Nestor 7-6(3), 4–6, 12-10 losing only to Robert Lindstedt and Horia Tecau in the Group Stage.

Nielsen finished the year as top Danish player in doubles, a year-end best No. 21 individually and No. 9 in ATP Doubles Team Rankings with Marray. He also earned a career-high $334,901 in 2012 courtesy of his win at Wimbledon.

Nielsen at the 2013 French Open

At the start of 2013, Nielsen partnered Johan Brunstrom for the first time since September 2012 at Metz, they reached the final of 2013 Heineken Open at Auckland losing out to Colin Fleming and Bruno Soares in two tough sets 6-7(1), 6-7(2). Dropped to 1-2 overall in tour-level finals. They lost in the first round of 2013 Australian Open to Simone Bolelli and Fabio Fognini 3–6, 4–6.

The Dane then partnered Bulgarian Grigor Dimitrov for World Tour Masters 1000 Miami and Indian Wells reaching second round of the former and Semi-Finals his first at Masters event losing out to Mariusz Fyrstenberg and Marcin Matkowski 4–6, 2–6. Played with André Sá in 2013 Power Horse Cup at Düsseldorf and fell in the Semis to Andre Begemann and Martin Emmrich. Along with Matkowski, reached Semis of 2013 Aegon International losing to Colin Fleming and Jonathan Marray 11–9 in Match Tie-Break. Played with Eric Butorac at the 2013 Winston-Salem Open, reached Semi-Finals where they lost out to Daniel Nestor-and Leander Paes 3–6, 2–6.

Nielsen partnered Johan Brunstrom and they entered the 2014 Aircel Chennai Open in January, the pair did not lose a single set on their route to the finals and went on to win the title defeating Marin Draganja and Mate Pavic 6–2, 4–6, 10–7. This was Nielsen's first title other than Wimbledon.

In January at Copenhagen during the Davis Cup Europe/Africa Zone Group II First Round Cyprus tie, he won both his singles and doubles matches partnering Thomas Kromann.

He then partnered Igor Sijsling for the 2014 Heineken Open and reached the quarters to only go down to the top seeds Alexander Peya and Bruno Soares 5–7, 5–7. He played at the Australian Open alongside Brunstrom and reached the second round before going down to Julien Benneteau and Edouard Roger-Vasselin in a tight three-setter 6–0, 5–7, 3–6.

Next he played at the Davis Cup Europe/Africa Zone Group II Luxembourg tie in April, winning both his singles and the doubles alongside Thomas Kromann, leading Denmark to a 5–0 victory, and to the Final Round in the Europe/Africa Zone Group II.

Partnering Brunstrom, he reached the semis at the 2014 Düsseldorf Open losing out to Martin Emmrich and Christopher Kas 0–6, 5–7. At the 2014 French Open the pair lost in the first round to Spaniards Marcel Granollers and Marc Lopez 5–7, 2–6. They next participated in the Wimbledon Championships falling in the second round.

In September, he played the Davis Cup Europe/Africa Zone Group II Denmark vs. Moldova tie, winning the first singles rubber and the doubles match but losing the reverse singles, thereby helping promote Denmark to the 2015 Davis Cup Europe/Africa Zone Group I.

He also won 3 ITF $15000 doubles titles at Switzerland, Dublin, Great Britain in March, July and October respectively with different and 1 ATP Challenger doubles title at Charlottesville Men's Pro Challenger in 2014.

Current through the end of 2021 Antalya Open.