The Socceroos know they have to win.

But the players' opinions are divided as to what will happen in the match being played simultaneously which could decide their World Cup fate.

If the Socceroos win over Serbia in Nelspruit on Wednesday (0430 Thursday AEST), they still are sweating on one of two results in the Germany-Ghana match in Johannesburg.

Either Ghana can beat Germany by any amount, or Germany must beat Ghana by a sizeable margin for Australia to progress to the round of 16.

Socceroos fullback David Carney sees Germany being able to smash Ghana to help balance out Australia's negative goal difference, so that even a one or two-goal win over Serbia would be enough to seal a second stage berth.

"Germany's very difficult. Even with 10 men they showed their class against Serbia," Carney said.

"In my opinion, Germany might win 4-0 or something, and hopefully we win one or two-nil. That should be just about right."

But midfielder Brett Emerton is plonking for the easier route - Ghana sneaking home over Germany to continue to run of upsets at the tournament.

"I think Ghana can (beat Germany). There's been bigger upsets already in this tournament than Ghana beating Germany," he said.

"But we have to do things at our end first."

While the opinions of what will happen in Johannesburg in the other match vary, one thing the Socceroos agree on is they can beat Serbia if they repeat their courageous performance in the weekend's 1-1 draw with Ghana which refloated Australia's World Cup.

"They're a good side, they've got good wingers and they're going to be very hard to stop, but I think they're definitely beatable," Carney said.

"We got the better of Ghana even with 10 men. Obviously they had a lot of the ball but the better chances went our way and hopefully we can do that against Serbia."

"It's definitely been a roller-coaster in this group.

"So far for us, we can holds our heads high and go into Serbia all guns blazing."

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