Before representing Croatia, he played for Bosnia and Herzegovina as a junior with partner Zachary Rosen.In 2004, Čilić played one Futures event (Croatia F1), at which he reached the second round; he finished the year tied at No.
Čilić came back from a break down in each of the second, third, and fourth sets.In the first round, he had defeated Julien Benneteau in five sets in a match that lasted more than four hours, winning the deciding set.He has a very powerful serve and very powerful groundstrokes.Čilić started to develop his career at a young age; his first steps of tennis started in Bosnia and Herzegovina.His fourth-round result at the Australian Open put him at no. Čilić also made it to the fourth round of Wimbledon, beating Jarkko Nieminen in five sets in the second round and knocking-out fourteenth seed Paul-Henri Mathieu.
At the 2008 Olympics, he reached the second round of the men's singles, beating Juan Mónaco before losing to Fernando González. At the Canada Masters, he defeated Andy Roddick in reaching the quarterfinal stage, giving his best performance in a Masters Series tournament to date.
He helped his country defeat Chilean opponents in the Davis Cup first-round tie by winning the opening singles match and joining forces with Mario Ančić to win the doubles rubber.
Čilić reached a career best at the French Open when he beat eighteenth seed Radek Štěpánek in the third round in straight sets.
Čilić won his first title of 2009 and second career ATP title in the 2009 Chennai Open, defeating first-time finalist Somdev Devvarman.
He reached the fourth round of the 2009 Australian Open after beating David Ferrer in straight sets, equalling his 2008 record.
In the singles, he was defeated by Mikhail Youzhny, who went on to win the tournament.