Novak Djokovic (Getty Images)
"I am backing Croatia... I know who I would like to lift the trophy," Serbian media quoted Djokovic as saying from Wimbledon.
Several weeks before the World Cup started Djokovic posted pictures of himself with Croatian players including midfield stars Luka Modric and Ivan Rakitic to his Instagram account.
When the Croatians faced off against hosts Russia in their quarter-final many Serbs who gathered to watch in cafes in central Belgrade were backing the Russians rather than their neighbours from the former Yugoslavia.
Djukanovic, on holiday in Greece, posted a video to social media in which he indicated that "those who say 'Nole' (Djokovic) has called for support for Croatia make me mad" and called the star an "idiot" for his stance.
"I repeat that all those who supported Croatia against Russia are psychopaths and mad, fit for being locked up in an asylum.
"Djokovic is a national hero and I thank him - but support Croatia, how can he not be ashamed? So many Serbs from Krajina (a region in Croatia where Serbs were once the majority) backed him and yet he supports the country who drove them out..." said Djukanovic.
President Vucic meanwhile explained that "I backed Russia as that is my right. Serbia is a democratic society and everyone has the right to support whoever he wants."
Croatia's reward for ending Russian hopes is a Wednesday semi-final in Moscow against England.