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. In January 2009, Klaasen was chosen as one of the two wildcards for the
2009 Premier League Darts tournament, where he competed against Taylor,
James Wade, Van Barneveld,
John Part,
Terry Jenkins, King and the other wildcard
Wayne Mardle. 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 reached the second round of the
2012 World Championship with a 3–1 win over
Peter Wright, but was beaten 0–4 by
James Wade in the second round. His best result of 2012 was in reaching the quarter-finals of the third
UK Open Qualifier in March where he lost 4–6 to
Mark Jones. At the
UK Open itself he was defeated in the last 64 by
Raymond van Barneveld 3–9. 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. He lost to
Michael van Gerwen 5–6 in the first round. 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. At the
UK Open he beat Gareth Pass,
Ross Smith and
Stuart Kellett to reach the last 16 for the first time since 2008. Klaasen played
Ronnie Baxter and lost 7–9. 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. 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. 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. 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.
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. 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. Klaasen was narrowly beaten 9–8 by
Andy Hamilton in the third round of the
UK Open. 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. 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.
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. 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. At the
UK Open, Klaasen beat
Brendan Dolan 9–4, before losing 9–5 to
Andrew Gilding in the fourth round. 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. 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. He was knocked out 10–4 by Whitlock in the first round of the
World Matchplay. In September, Klaasen's second title of the year came at the 16th Players Championship with a 6–4 victory over
Raymond van Barneveld. 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. The match went all the way to a deciding leg in the fifth set in which Klaasen crucially missed five opening doubles and could never catch up to lose 3–2. 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.
2016 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. 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. In the semis he was thrashed 6–0 by Anderson, who averaged 107. 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. 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. Klaasen knocked out
Dave Chisnall 6–4 to reach the final of the
German Darts Championship, but lost 6–5 to Norris after leading 4–2. 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. He missed three match darts in the third round of the
Players Championship Finals as Chisnall edged through 10–9.
2017 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. 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. He lost 6–2 in the final of the
German Darts Masters to Van Gerwen.
2018 At the
2018 PDC World Darts Championship, Klaasen was defeated by
Jan Dekker 3–1 in the first round. He was defeated 10–5 by
Peter Wright in the opening round of the
World Matchplay.
2019 At the
2019 PDC World Darts Championship, Jelle was defeated by
Keegan Brown 3–1 in the second round. He lost in the fourth round of the
UK Open to
Madars Razma 10–4. He then qualified for his first euro tour of the year
2019 International Darts Open, losing in the first round to
Joe Murnan 6–5. He then lost in the
Players Championship Finals to
Max Hopp 6–3.
2020 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. At the
2020 UK Open, he started his campaign by defeating Riley's qualifier
Jason Askew 6–4. Klaasen then beat
William Borland 10–5 before thrashing
Steve West 10–1 to set up a sixth-round clash with
Gary Anderson. He overcame Anderson 10–9 before losing in the quarter-finals to
Daryl Gurney.
2021: Loss of Tour card 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.
2022-2023: WDF Career From the beginning of 2022, Klaasen took part in tournaments organized by the
World Darts Federation. In June 2022, he took part in the
Dutch Open, where he was seeded in 26th. On his way to the final of this tournament, he defeated leading WDF players including
Richard Veenstra and
Thibault Tricole, before defeating
Mark Barilli in the final. This qualified him for the
2023 WDF World Darts Championship and
2022 Winmau World Masters. In November, he won the
Irish Open. On his way to the final, he defeated
Michael Warburton,
Kai Fan Leung and
Ryan Hogarth. In the final, he faced Dylan Slevin and defeated him by 6–3 in legs. He ended the 2022 season as the leader of the
World Darts Federation men's ranking.
2024–2025: Return to PDC 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. In interviews, Klaasen reflected positively on his WDF experience but emphasized his readiness to compete at the highest level again. Klassen reached his first
PDC Pro Tour final since his return at
2025 Players Championship 3, being defeated by
Chris Dobey 8–4. ==Personal life==