"Only one person firmly believed that Russia would beat Spain. [It was] Aleksandar Vucic. His optimism during our conversation on June 29 was striking. And he was right! Thanks to all our Serbian friends and fans," the ambassador said on his Twitter account.

On Sunday, hosts Russia beat 2010 World Cup winners Spain on penalties to reach the quarter-finals for the first time since the break-up of the Soviet Union in 1991.

Russia will now face Croatia at the Fisht Stadium in Sochi on Saturday.