BELGRADE (AFP) - Serbian tennis star Novak Djokovic has come under fire for voicing support for bitter rivals Croatia at the World Cup, with one centre-right lawmaker dubbing his stance "idiotic".

"Only idiots can support Croatia. Aren't you ashamed Novak?", tweeted Vladimir Djukanovic of President Aleksandar Vucic's ruling Serbian Progressive Party.

"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 on Instagram pictures of himself with Croatian players including midfield stars Luka Modric and Ivan Rakitic.

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.

Mr 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 (July 11) semi-final in Moscow against England.