After winning a silver medal in the 50 m breast and a bronze medal in the 100 m breast at the European Junior Swimming Championships 2004 in Lisbon, Portugal. Stekelenburg made his international senior debut at the 2005 World Aquatics Championships in Montreal, Quebec, Canada where he was eliminated in both 50 m and 100 m breaststroke. In the following years he swam several European Championships making his best performance at the 2008 European Aquatics Championships in Eindhoven in his home country the Netherlands, ending up 6th in the 50 m breaststroke final after swimming a national record in the semi-finals.