DUSSELDORF, Germany - Captain Michael Ballack admits Germany has work to do on the defensive side to ensure success in the World Cup.

Coach Jurgen Klinsmann's youthful backline looked vulnerable on countless occasions in a 2-2 friendly against Japan. Two late goals salvaged a tie for the Germans, but that was not enough to satisfy Ballack, who called on his teammates to pick up their game on defense.

"We need to be more concerned about our defense," the talented midfielder said. "That game (friendly against Japan) showed we still have not sorted out the same problems we have been having for the last two years. It is our offensive philosophy to play quickly to the front but we are losing too many balls."

Ballack, 29, also admitted he disagreed with Klinsmann's decision to play him out of position in the Japan match. The Chelsea star spent most of the time on the right of midfield to grant Tim Borowski additional room in the center.

"I expect to play in my central role again in the tournament," said Ballack, who has been bothered by a slight calf strain but played in the final two friendlies against Japan and Colombia.

Germany assistant Joachim Low defended the coaching staff's decision to experiment in midfield.

"If Ballack only plays centrally, it's easier for the opponents to cover him, so we thought it would be good to vary his positions," Low said. "There are other players, like Ronaldinho (of Brazil), who also move places in midfield and then rush to the front."

Low refuted rumors about a potential rift between Ballack and Klinsmann over the coach's experimental lineups.

"There is absolutely no discrepancy over our philosophy," Low said.

But the World Cup is off to a shaky start for Germany. Ballack looks almost certain to miss the opener against Costa Rica after pulling out of Wednesday's practice session. He had not trained since straining his right calf in last Friday's 3-0 friendly victory over Colombia.

Ballack returned Wednesday but was forced to end the session early after his injury would not allow him to participate in the final training match.

For decades, Germany's superior conditioning enabled it to beat more talented teams. Including its results as West Germany, no nation can match its record of reaching the semifinals in nine consecutive World Cups (1954-90) or playing in seven finals.

Ballack stands alone among modern German players for his remarkable talent. In the 2002 World Cup, Ballack scored the goal that enabled Germany to beat the United States in the quarterfinals.

Against co-host South Korea in the semifinals, Ballack deliberately thwarted an opposing attack with a foul, fully aware that his transgression would yield a yellow card and suspension from playing in the final.

Legendary German player and former coach Franz Beckenbauer praised Ballack's "sacrifice." However, Ballack's absence certainly hurt Germany in its loss to Brazil in the final.

A rematch against Brazil in the 2006 final is a possibility, but Germany clearly needs to tighten up on defense.