His first attempt at the international stage was a 2005 Winmau World Masters ended with a 2–3 defeat to Irishman Steve McDonald.
On 15 January 2006, Klaasen became the youngest World Darts Champion, aged 21 years 90 days, when he defeated fellow Dutchman Raymond van Barneveld in the final of the 2006 BDO World Championship by 7 sets to 5. He also beat Shaun Greatbatch and world number one Mervyn King on his way to the final. Before the championship, he was said to be a 100–1 outsider to win the title as he did not even have a BDO Ranking.
Klaasen has won some Open events in his native country including the Alkmaar Open. In March 2006 he was runner-up in the Finnish Open, losing in the final to Martin Atkins by 2–6. He also reached the quarter-finals of the Swiss Open and Denmark Open during 2006. 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 returned to the Lakeside to defend his World title at the 2007 Championship. 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.[2] Although Klaasen was lower in the rankings at the time, it was considered an upset as Stompé had not won at the Lakeside in the previous four years.
Klaasen did not make an immediate impact on the PDC circuit, having to start virtually from scratch at 262 in the world rankings. He competed at most of the Pro Tour events (Players Championships and UK Open Regional Finals) to improve his ranking. 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 only made it into one televised PDC event in the UK during 2007 (the UK Open where he reached the last 32) but his form towards the end of the year ensured automatic qualification for the 2008 World Championship – his debut in the PDC version. 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. In the quarter finals, he was beaten by Van Barneveld 5–1. The match is best remembered for Barneveld hitting the first 9-dart leg in the history of the PDC World Championship.
Klaasen found life hard in the Premier League, losing 5 and drawing 2 of his opening 7 matches. On 26 March, Klaasen defeated Terry Jenkins 8–6 to finally record his first victory in the competition. Klaasen finished 7th in the Premier League after Wayne Mardle withdrew from the competition. Klaasen qualified for the World Matchplay in Blackpool but lost 10–8 to Ronnie Baxter in the first round.
Klaasen lost in the first round of the 2010 World Championship losing 3–1 to Welshman Barrie Bates. He then reached the quarter finals of the 2010 World Matchplay, beating Mervyn King and Gary Anderson before losing to Simon Whitlock.
Klaasen returned to darts in January 2013 by qualifying for the first European Tour event of the year, the UK Darts Masters, with a 6–2 win over Carlos Rodríguez.[8] He lost to Michael van Gerwen 5–6 in the first round.[9] 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. He averaged 103 in the final against Jamie Caven but lost 4–6 with his opponent averaging almost 111.[10] At the UK Open he beat Gareth Pass, Ross Smith and Stuart Kellett to reach the last 16 for the first time since 2008.[11] Klaasen played Ronnie Baxter and lost 7–9.[12] 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.[13] 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.[14] Klaasen was outplayed by Gary Anderson in the first round of the World Grand Prix as the Scotsman did not give Klaasen one dart at a leg winning double in a 2–0 in sets ten minute match. In the double-start event Anderson averaged 104.86, almost 40 points ahead of Klaasen's 67.22.[15] Klaasen knocked out number two seed Dave Chisnall 6–4 in the Players Championship Finals, before losing 9–5 to Anderson in the second round.[16]
2014
Klaasen played Jamie Caven in the first round of the 2014 World Championship and lost 3–1 as he hit just 17% of his darts at doubles.[17] 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. He played Justin Pipe and was denied a place in the final as Pipe took out a 134 finish in the deciding leg.[18] Klaasen was narrowly beaten 9–8 by Andy Hamilton in the third round of the UK Open.[19] 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.[20] Klaasen averaged 108.74 in beating James Wade 6–4 in the first round of the Players Championship Finals, but then averaged nearly 20 points lower as he missed 22 doubles in the next round against Dean Winstanley to be beaten 10–5.[21][22]
2015
Klaasen met Gary Anderson in the second round of the 2015 World Championship after seeing off compatriot Christian Kist 3–1. Klaasen went 3–1 up and was two legs to one ahead in the next set, before Anderson took out 84 on the bull with Klaasen waiting on 89 for the match. Anderson sealed the set in the next leg and won the final two sets by three legs to one scorelines to win 4–3. Both players averaged over 100 with 20 180s thrown during the match.[23] Klaasen defeated Phil Taylor 6–2 at the fourth UK Open Qualifier to play in his first final in almost two years. However, he missed 10 match darts to slip from 5–2 ahead of Michael van Gerwen to lose 6–5.[24] At the UK Open, Klaasen beat Brendan Dolan 9–4, before losing 9–5 to Andrew Gilding in the fourth round.[25] 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.[26] Klaasen reached his third final of the year at the 14th Players Championship and won his first title in nearly eight years by beating Ian White 6–2.[27] He was knocked out 10–4 by Whitlock in the first round of the World Matchplay.[28] In September, Klaasen's second title of the year came at the 16th Players Championship with a 6–4 victory over Raymond van Barneveld.[29]
Klaasen overcame Joe Cullen and Mervyn King to advance to the third round of the World Championship for the first time since 2009. He missed one dart at double 18 to complete a 147 finish and defeat Phil Taylor 4–2, before going 2–0 down in legs in the final set. Taylor missed one dart to win the match and Klaasen took full advantage by winning four legs on the trot, wrapping it up with an 11 darter.[33] In the quarter-finals, Klaasen was 3–1 and 4–3 behind Alan Norris in a race to five. He was two legs to one down in the next set but produced back-to-back 11 dart legs to win it and then clinched the final set without conceding a leg to advance to the semi-finals of a PDC World Championship for the first time, where he faced reigning champion 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.[34][35] In the semis he was thrashed 6–0 by Anderson, who averaged 107.[36] 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.[37] He was defeated 10–6 by Steve Beaton in the opening round of the World Matchplay and at the same stage of the World Grand Prix, 2–1 in sets by Adrian Lewis.[38][39]
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 took the next set and led by two legs to none in the sixth, before Chisnall won three legs on the spin to triumph 4–2.[43] His return to the Premier League was blighted by tendinitis in his wrist for which Klaasen received anti-inflammatory injections before matches. Klaasen explained how he could play for five or six legs before experiencing pain in his throwing hand. His only win out of his nine league games came against Chisnall and Klaasen finished ninth in the table to be eliminated.[44] He lost 6–2 in the final of the German Darts Masters to Van Gerwen.[45]
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. Following this, Klaasen participated in various non-Tour Card events, including the PDC Challenge Tour and WDF competitions.
Klaasen successfully regained his PDC Tour Card during the European Q-School in 2024 after a two-year absence from the professional circuit. His path included a series of tightly contested matches in the final stage of Q-School, culminating in a victory over Benjamin Drue Reus in the decisive match.[47] In interviews, Klaasen reflected positively on his WDF experience but emphasized his readiness to compete at the highest level again.[48][49]
Before the 2006 World Championships, Klaasen worked for a manpower agency in the Netherlands but he has now taken up darts full-time. He is sponsored by Keukenconcurrent, a company that produces fitted kitchens in his home country.[citation needed]
Controversy in 2012
In August 2012, Klaasen was accused of sending naked photo messages of himself to a then 15-year-old family friend of his compatriot – fellow darts player and former friend since childhood – Michael van Gerwen,[50] which he has admitted to in the Dutch press.[51]
According to her mother, the underaged girl was diagnosed with PDD-NOS, an autism spectrum disorder. Though during the prosecution service process (the case didn't go to court) this was never proven.[52]
Later that year, on 13 December, he was sentenced to 16 hours of community service and a fine of €327, to be paid to the girl.[53]
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