In 2001, Feijen reached the finals of a nine-ball tournament in Tokyo, Japan. The event had a field of more than 700 players and offered the largest prize money at that time. However, he lost to Efren Reyes. In 2004, he won the inaugural Skins Billiards Championship with prize money of US$42,500. Feijen has won the European straight pool championship four times. In 2005 he was the winner of the Big Apple Nine-ball Classic, held in Queens, New York, an event with 128 of the world's best players. He represented Europe in the 2001, 2004, 2005, 2007-2009, 2011 and 2012 Mosconi Cup events.
More recently, Feijen won the 2007 $50,000 winner-take-all International Challenge of Champions by defeating Lee Van Corteza. In 2008, Feijen won the World Straight Pool Championship with a victory over Francisco Bustamante in the finals. On October 5, 2008, he received the 3rd prize of $25,000 in the inaugural WPA World Ten-ball Championship.
In 2010, Feijen reached the finals of the WPA World Eight-ball Championship but ended up in 2nd place as he was dominated by Karl Boyes of Great Britain. He would reach the finals of that same tournament for the second consecutive time in 2011 but only to be defeated by Dennis Orcollo of the Philippines.
- insidepoolmag.com, Niels Feijen
- insidepoolmag.com, Finals at World 10-Ball Championship Set
- "REYES POCKETS POOL'S BIGGEST PAYOUT". Billiards Digest. Retrieved 2008-08-08.
- "By the Skins of His Teeth: Feijen Wins $42,500 at Skins Event Debut". AZ Billiards. Retrieved 2008-08-07.
- "Sports 123: Pool: European Championship: Men: 14.1". Sports123. Retrieved 2008-08-08.
- "Feijen Beats Corteza, Wins US$50,000 in International Challenge of Champions". AZ Billiards. Retrieved 2008-08-07.
- "Feijen is Predator World 14.1 Straight Pool Champion". Inside Pool Magazine. 2008-08-30. Retrieved 2008-08-31.
- billiardsdigest.com "Rule, Britannia! Appleton Wins World 10-Ball Championship"
- sports.inquirer.net, "Appleton Nips Wu for Title; Pulpul 4th"
- Jerry Forsyth (April 10, 2010). "World 8-Ball Finals". AzBilliards.com. Retrieved 2010-04-11.
- WPA Staff (February 26, 2011). "Orcullo Wins". AzBilliards.com. Retrieved 2011-02-26.
-  WorldPoolMasters.com 2006 profile of Niels Feijen
-  AzBilliards.com August 14, 2005 report on Niels Feijen's win of the 2005 Big Apple Nine-ball Classic
- Niels Feijen 2008 World 14.1 Championship Winning Video
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