The world and European champions top Group I with three wins from their first three games, but Del Bosque feels Michal Bilek's side pose a major challenge.

"They are our biggest threat," Del Bosque said. "They have great physical and technical qualities. They are very strong and have been one of the best teams in Europe in recent years.

"The worry for us is that most of our players are involved in domestic and European campaigns with their clubs and will continue to be so until the end of the season.

"We need to go into Friday's game with the right mentality because if any one of us is switched off, it could really complicate things."

Striker Fernando Torres is certainly sounding the right notes, telling El Mundo Deportivo that the players are focused on retaining their European title.

"The difficult thing in football for national teams is learning to win," the Chelsea player said. "Spain did after many years of disappointments.

"There is now another challenge to retain the European Championship, something that nobody has ever done before."

Valencia midfielder Juan Mata could get a rare start on Friday and is looking forward to the game, despite the distraction of the latter stages of the league campaign in Spain.

"Being selected is very important and takes all our focus now," he said. "In La Liga it is the most crucial time of the season but that's after the game against Lithuania."

The Czech Republic are second in the group with two wins from three games, but have scored just three goals in the process, two of them against minnows Liechtenstein.

Galatasaray striker Milan Baros returns to the squad and Bilek said: "His club are not in the best shape but I am sure he will be very useful."