Jelusick wrote his first song in English at the age of seven. In 2003, he participated in the first Junior Eurovision Song Contest 2003 in Denmark with a song he wrote at the age of ten, "Ti si moja prva ljubav", and won with 134 points. Jelusick released his debut solo album, "No. 1", in 2003, and an English version in 2004. He toured the United States, Scandinavia, and Australia until 2005, and the tour included four large concerts in Denmark with a total audience of 100,000 people. The same year, Jelusick became the youngest nominee of the Croatian music award Porin, at the age of twelve. In 2005, he was one of the three main acts at the Langeland festival in Denmark, along with UB40 and Ronan Keating.