[2] He competed in the World Cup for the first time in Kuusamo 2003, but was disqualified from both races because his skis were too long compared to his length and weight (BMI rule).

[citation needed] In February 2004 he won a bronze medal at the Junior World Championships in Stryn.

In March 2004, at the very end of the 2003–04 season, Koivuranta made his proper debut in the World Cup circuit—in two races in Lahti, he finished 24th and 23rd respectively.

[2] In December the same year, in early 2004/05 season, he finished among the top ten for the first time, with an eighth place in the sprint race in Trondheim.

At his last Junior World Championships, held in Tarvisio in March 2007, he won a gold and a silver medal.

By 2008/09 season Koivuranta had become one of the better cross-country skiers as well, in part thanks to a newly appointed coach to the Finnish national team.

[2][4] The World Championships in Liberec 2009, was not successful for Koivuranta as he did not win any medals at all, and his best results were two fourth places .

Koivuranta was also sick during the World Championships which affected his results and that he was unable to compete in the team competition.

The season as a whole was a disappointment for Anssi who struggled with persistent sickness throughout the winter and as a result, failed to match the successes of his previous championship year.

This would also prove to be Koivuranta's final season in Nordic Combined before his transition to ski jumping.

The disease prevented him from doing any physical endurance practice which resulted in his decision to fully concentrate on ski jumping.