Germany captain Michael Ballack missed another training session because of his calf injury on Tuesday, heightening concerns that he may not be fit for the World Cup opener against Costa Rica on Friday.

Ballack had said he expected to be back training with the ball on Tuesday afternoon after coming through an indoor workout in the morning but that proved to be impossible.

"Michael is making progress but (the calf) can still not take a lot of pressure," Germany doctor Hans-Wilhelm Mueller-Wohlfahrt said in a statement.

"We hope that on Wednesday he can take a full part in training again."

Ballack had earlier said he expected to be in the team against Costa Rica in the Group A curtain-raiser in Munich.

"I think it will continue improving day by day and I assume I will be able to play on Friday," he said.

Ballack suffered the injury in Germany's final warm-up match for their home tournament, a 3-0 win over Colombia on Friday.

"I know deep down that it's not a bad injury so I didn't do anything about it on Sunday, which was my one day off to spend with my family," said Ballack.

Germany's assistant coach Joachim Loew sounded a more cautious note.

"We hope that after a little rest and the work of the medical department he will be fit," he said. "It's just a little calf muscle problem but it is a nagging injury."

He added: "It has hurt the atmosphere here a little."

Ballack's inspirational displays helped Germany reach the World Cup final in 2002 when they lost to Brazil. He has scored 31 goals in 65 internationals from midfield and his form and fitness are of vital importance to the hosts.

Germany coach Juergen Klinsmann would likely call on Tim Borowski as Ballack's replacement in central midfield.