He also played in the 2006 International Darts League and beat 13-times World Champion Phil Taylor 7–6 in his last group match, but it was not enough to qualify for the knockout stages.

He beat Andy Fordham and Dick van Dijk but lost to Martin Adams, Shaun Greatbatch and Colin Lloyd in his other group matches.

In 2006 Winmau World Masters, he was seeded 13 but suffered a straight sets defeat in his first match against fellow Dutchman Mario Robbe.

Klaasen was unseeded for the event after a poor run of form and was facing the seeded Co Stompé, with eventual winner Martin Adams waiting in round two should he win.

Klaasen became the sixth defending champion to fall at the first hurdle in his title defence with his first round loss to fellow countryman Stompé by three sets to nil.

He reached the quarter-finals of two non-televised Pro Tour events before winning a Players Championship – the John McEvoy Gold Dart Classic in November which saw him climb to 49 in the PDC Order of Merit.

He lost in the first round to fellow Dutchman Vincent van der Voort in a sudden death leg, having missed ten darts to win the match.

He defeated Colin Lloyd, Jan van der Rassel and Andy Hamilton to reach the quarter finals of the 2009 PDC World Championship, only dropping one set in the process.

He then reached the quarter finals of the 2010 World Matchplay, beating Mervyn King and Gary Anderson before losing to Simon Whitlock.

[8] After revelations emerged with regard to Klaasen's personal life detailed below he did not play again in the rest of 2012, which included missing the 2013 World Championship.

[10] In May, Klaasen reached his first PDC final since November 2007 at the fourth Players Championship by beating the likes of Mervyn King and Gary Anderson.

[13] He beat Wes Newton 6–4 and Justin Pipe 10–6 to advance to the quarter-finals of the European Championship, where he was defeated 7–10 by Michael van Gerwen.

[14] In the first round of the World Matchplay he came back from 3–8 down against Simon Whitlock to trail only 7–9, but lost the next leg to fall short of a successful fightback.

[18] At the German Darts Championship he reached the semi-finals of a European Tour event for the first time by defeating Michael van Gerwen 6–5 in the quarters.

[20] At the European Championship, Klaasen knocked out reigning champion Adrian Lewis 6–4 in the first round and Caven 10–7 to reach the quarter-finals for the second year in a row, where he lost 10–5 to Mervyn King.

[26] Klaasen recorded successive wins over Adrian Lewis, Mervyn King and Simon Whitlock to reach the final of the 12th Players Championship where, after taking the opening leg, he was beaten 6–1 by Peter Wright.

[30] Klaasen reached his first quarter-final at the World Grand Prix by eliminating Brendan Dolan 2–1 and Dave Chisnall 3–2 and faced Mark Webster.

[32] Wins over Raymond van Barneveld and James Wade saw Klaasen reach another major quarter-final at the European Championship, but he was defeated 10–6 by Gary Anderson.

Klaasen's victory also marked the first time that a player has won their next game after knocking Taylor out of a PDC World Championship.

[37] Klaasen also reached the semi-finals of the next major event as he whitewashed Kyle Anderson 10–0 at the UK Open, but he lost 10–5 to Peter Wright.

[41] 6–3 and 10–4 wins over Steve West and Stephen Bunting saw Klaasen face Michael van Gerwen in the quarter-finals of the European Championship and he was comfortably beaten 10–3.

[43] He got to the third round of the World Championship for the second year in a row with comfortable victories over Jeffrey de Graaf and Brendan Dolan, but found himself 3–1 down to Dave Chisnall.

Klaasen beat Kevin Burness 3–1 in the first round of the 2020 PDC World Darts Championship, before losing to Michael van Gerwen by the same scoreline.

Klaasen lost his PDC Tour Card at the end of the 2021 season after struggling to maintain consistency in his performances after being a fixture on the professional darts circuit for 14 years.