Croatia, who beat Denmark 3-2 on penalties in the round of 16, will face Russia in the quarter-finals at Sochi's Fisht Stadium on Sunday (4am AEST).

"Here they are in the quarter-finals, where nobody expected them to be," Modric said on the tournament's official Twitter account.

"They deserved that if you ask me. We expect a difficult match against them. We'll have to be much better than we were against Denmark, which I'm confident we will be."

Modric added that the support of fans had helped Russia get through to the quarter-final beating Spain, who were considered to be favorites to win the match.

"They are the host team, very strong, they work hard and fight as team," he said.

"They showed that against Spain, they were underdogs with very few chances. In the end they survived the penalties, with the help of the home fans. They run and fight a lot, they were rewarded for that."

The 2018 FIFA World Cup kicked off on June 14 and will continue until July 15, with the final set to be held at Moscow's Luzhniki Stadium.