Nicknamed "The Real Deal", he won his first major title at the 2024 World Grand Prix, defeating Luke Humphries 6–4 in the final.

In Autumn, De Decker was able to take part in Players Championship events for the first time as a call-up from the Challenge Tour virtue of his ranking on that Order of Merit.

He reached the last 16 of the 2017 Dutch Darts Masters on the European Tour for the first time after defeating Richard North and Alan Norris and also qualified for the 2017 Players Championship Finals, where he was eliminated in the first round by Adrian Lewis.

After a win on the Development Tour at the end of the year, De Decker again qualified for the 2017 PDC World Youth Championship and this time reached the last 16, where he was eliminated by reigning champion Corey Cadby.

De Decker also took part in several European Tour events and reached the last 16 of the 2019 Austrian Darts Championship after defeating Chris Dobey and Jonny Clayton, which he narrowly lost to Rowby-John Rodriguez.

In January 2020, he won a Tour card at the European Q-School in Hildesheim, beating Dennis Nilsson 5–3 in the Day 2 final.

De Decker qualified for the 2023 PDC World Darts Championship, where defeated Jeff Smith 3–1 in the first round.

De Decker won his maiden PDC senior title by defeating Ricky Evans in the final at Players Championship 16.

In October 2024, De Decker won his first ever PDC major title, defeating Luke Humphries 6–4 to win the 2024 World Grand Prix in only his second appearance in the tournament.

Speaking on Sky Sports News, PDC chief executive Matt Porter commented that De Decker would not be ready for the Premier League as he had not frequently played in the latter stages of televised tournaments, but described him as an "outstanding talent" and opined that De Decker would likely be in the Premier League in the future.