Spain defeated Iran 1-0 in their second Group B encounter in Kazan on Wednesday. The winning goal was scored by forward Diego Costa in the 54th minute.

"It was very difficult for everyone, but we have three points, that's what matters," Hierro said.

"We started well, but then we lost our rhythm because of frequent pauses. But we completed our mission after the halftime break, so I'm happy. I hadn't expected that the match against Iran would be so difficult."

Hierro added that this summer's World Cup wouldn't be easy for Spain.

"We will draw conclusions and continue playing as a team. It will be a very difficult tournament. There will be no easy matches, but we believe in the way we've chosen and will develop," he said.

Spain will face Morocco in their last group game on Monday.

The 2018 FIFA World Cup kicked off on June 14 and will continue until July 15, with the matches set to be played in 11 cities across Russia.