haapoja made his professional debut on 8 april 2006, losing a four-round points decision to remigijus ziausys, who also debuted. a ten-round split draw against jari markkanen on 16 march 2007 denied haapoja a chance to win the finnish heavyweight title. he lost for a second time to denis bakhtov on 22 may 2008, which was for a wbc and paba regional heavyweight title. on 28 november 2008, haapoja stopped sami elovaara in nine rounds to become the finnish heavyweight champion, winning the vacant title in his second attempt. a defence of this title against jarno rosberg, which went the ten-round distance, was later relegated to a no contest after rosberg was caught for steroids.