Socceroos hardman Vinnie Grella has warned rivals Germany that Australia has the special players to cause them World Cup grief.

Grella senses the big-match players in the Socceroos camp - Tim Cahill, Harry Kewell and Mark Bresciano - can fire Australia to a huge World Cup opening game upset in Durban on Sunday.

While questions remain over just how fit two of them - Cahill and Kewell - might be going into the clash, Grella believes they are ready to shine on the sport's biggest stage.

Grella says the trio, who have all scored important goals in crucial Socceroos matches and excel in pressure games, will be the key to Australia making the best possible World Cup start.

"You need the special players in a team - and we have a couple of them in Bresciano, Cahill and Harry Kewell if he's fit - these types of players will make the difference," Grella said.

"Our job is to put them in the position to score goals. They'll be the key differences.

"We have some boys that have got some steel and we have some boys that have got class.

"If we make them hit together in the right spot on the pitch, we will put the Germans under a lot of pressure."

Kewell has played just a few minutes of football since January because of his groin, but looks certain to play some role against Germany after again training strongly on Tuesday night.

More of a worry is Cahill, who completed just the warm-up and light work before sitting out the practice match which followed.

The Everton midfielder injured his neck in Australia's 3-1 friendly loss to the United States on Saturday, and was taken to hospital for observation.

Socceroos coach Pim Verbeek said scans showed no serious injury, but Cahill is yet to train fully since that match.

With the Australians holding a behind-closed-doors training session on Wednesday - the guessing game over Cahill's health will continue.

Cahill did little to hide his disdain for questions about his fitness when quizzed by media.

"I don't want to stand here talking about injuries and questions all the time about injuries. I just want to get on with it," he said.

As Australia's most important strike weapon going into the match with a goal every two games for his country, the Socceroos can ill afford Cahill being under any sort of cloud.

Germany defender Per Mertesacker admitted Cahill is top of his side's hit-list as they plan to limit his impact on Sunday.

"Tim Cahill has enormous experience and has grown tremendously in England. He is one of Australia's most dangerous players and will be their midfield maestro," Mertesacker said.

"But if we can mark him tightly, it will make it hard for him to play a role."

Midfielder enforcer Grella has been in patchy form since arriving in camp, though he has been struggling with a cold since the US friendly.

He admitted the team had been given a reality check by their loss to the US - the Socceroos' first in eight matches and just the fifth in Verbeek's 30-match reign.

But he said the team's tough training regimen ahead of the match could have had some effect - something the team had been warned about prior to their two friendlies in South Africa.

"We've had a small slap in the face against the US ... there's some detail we can improve on," Grella said.

"But with the next week we can iron them out."

The Socceroos' other injury concern, midfielder Brett Emerton, trained well on Tuesday and looks set to play against Germany.

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