Raised in Lengel, Gelderland, he participated in football and long-distance running as a child. He gradually became more interested in running and signed up with Running Team Achterhoek, a local group. He competed in national junior competitions and began to train regularly around 2008. He placed in the top three in the Dutch junior 800 m race in 2009 and 2010.
Kupers began to make an impact nationally in 2012. He set a personal best of 1:48.71 minutes to win the indoor Dutch title, then set a best of 1:46.46 minutes at the FBK Games, which ranked him second among Dutch athletes behind Robert Lathouwers. He was selected for the 2012 European Athletics Championships and made the semi-finals at the competition – his first senior international event. He defended his national indoor title in 2013 and went on to run in the semi-finals at the 2013 European Athletics Indoor Championships. Outdoors, he came fifth at the European Team Championships first league race, then sixth at the 2013 European Athletics U23 Championships, where he was also seventh in the 4×400 metres relay. He won his first Dutch outdoor title in Amsterdam that season.
|2012||European Championships||Helsinki, Finland||8th (semis)||800 metres||1:50.37|
|2013||European Indoor Championships||Gothenburg, Sweden||6th (semis)||800 metres||1:50.29|
|European Team Championships||Dublin, Ireland||5th||800 metres|
|European U23 Championships||Tampere, Finland||6th||800m||1:47.15|
|2014||World Indoor Championships||Sopot, Poland||5th||800 metres||1:47.74|
|2015||European Indoor Championships||Prague, Czech Republic||3rd||800 metres||1:47.25|
|World Championships||Beijing, China||15th (sf)||800 m||1:47.74|
|2016||European Championships||Amsterdam, Netherlands||6th||800 m||1:46.67|
|2017||European Indoor Championships||Belgrade, Serbia||5th||800 m||1:50.47|
|World Championships||London, United Kingdom||1st (h)||800 m||1:45.531|
1 Din not start in the semifinals
- National titles
- 800 metres indoor: 2012, 2013, 2014
- 800 metres outdoor: 2013
- Thijmen Kupers. Running Team Achterhoek. Retrieved on 2014-03-10.
- Biographie (in Dutch). Thijmen Kupers. Retrieved on 2014-03-10.
- 2012 FBK Games Results - 800m Men . FBK Games. Retrieved on 2014-03-10.
- Thijmen Kupers. Tilastopaja. Retrieved on 2014-03-10.
- Jalava, Mirko (2014-03-09). Report: men’s 800m final – Sopot 2014. IAAF. Retrieved on 2014-03-10.
- Thijmen Kupers - IAAF Athlete Profile.