Joachim Low's side are the current holders of the trophy, having beaten Argentina 1-0 in extra time in the final in 2014.
Despite being tipped to retain their title in Russia, Kroos has played down the pressure and insists that his international side are "not as dominant" as they once were.
"Clearly, [winning the World Cup again] is not that easy a task. Only two teams have done it in the last 20 World Cups. That tells the real story," told reporters.
"The most difficult task is to defend the World Cup successfully. We need to improve and play at our best like we did in 2014 to achieve it. Things have changed — both in our team as well as in other teams. We have played against a few big teams and we were not as dominant. That gave us a clear picture. Other teams had improved too in this period of four years - the Brazil of 2018 is much better than the team from 2014. We are one of the favourites, undoubtedly, but not the overwhelming favourites.
"Traditionally, big teams tend to play better during the World Cup. Spain and Brazil are two of the most talented sides. Any others? France, Portugal, Belgium, England and Argentina. We shall be missing Italy. The World Cup and Italy seemed to be inseparable. It will be a disappointment for the Azzurri fans."
Germany will play Mexico, Sweden and South Korea in Group F of the World Cup later this month.
World Cup 2018 betting sites are offering odds of 9/2 for Germany as outright winners of the tournament.