Germany captain Michael Ballack missed training as a precaution today, four days before the start of the World Cup, after straining his right calf muscle.

Ballack remained at the team hotel for a solo fitness session, a spokesman for the German Football Association said.

"It's a precautionary measure because the players will be doing sprints in the main session," the spokesman said.

Germany play Costa Rica in the opening match of the World Cup in Munich on Friday.

Ballack, who recently signed for Chelsea, has scored 31 goals in 65 internationals and his form and fitness are crucial to Germany's chance of a home success in the tournament, which runs until July 9.

The 29-year-old midfielder was also forced to miss training for several days at the squad's recent camp in Geneva.

The problem then was an ankle strain and he was unable to play in the friendly against Luxembourg, although he returned to face Japan and Colombia in the final two warm-up games.

Germany otherwise have a clean bill of health as they prepare to get the World Cup underway.

The other 22 players all trained on Monday, the first session since the squad reassembled at their Berlin base.

Germany will also play Poland and Ecuador in Group A.