Germany captain Michael Ballack was unable to finish a training session on Wednesday and is a major doubt for the World Cup opener against Costa Rica.

The 29-year-old Chelsea midfielder was attempting to complete a first practice session since sustaining a calf strain in last Friday's 3-0 warm-up victory over Colombia.

However, he was forced to end the session early and was unable to take part in the final training match.

The German FA announced the former Bayern Munich star will "receive intensive medical treatment in the next few hours", but head coach Jurgen Klinsmann has previously insisted he will not play anyone "who is not fit 48 hours before a match".

If Ballack fails to make it in time for Friday's encounter, Werder Bremen midfielder Tim Borowski is expected to take his place.