The injury-hit Wallabies are confident tough hooker Tatafu Polota-Nau will tackle the USA in Friday night's must-win Rugby World Cup clash despite sending another scare through the camp.

Polota-Nau took a tumble at Thursday's captain's run and finished the session with ice on his knee.

But the Wallabies - already down to two hookers - are optimistic he'll be fine.

"He's a pretty tough cookie, old Taf," prop Ben Alexander said on Thursday.

"Not too many games go by where he takes a beating. He puts his body on the line even at training.

"He'll be fine. I think it's just precautionary. He's pretty good with his rehab and everything, so he just ices his knee after any session just to keep on top of it."

Polota-Nau played his first Test of the year last Saturday, being rushed into the starting side as a match-day replacement for the ailing Stephen Moore, after missing the entire Tri Nations tournament with a knee injury.

The 26-year-old only proved his fitness for the World Cup in the Australian Barbarians' win over Canada the week after Robbie Deans announced his 30-man squad for New Zealand.

With Saia Faingaa out with the flu, the Wallabies will be in a real bind if Polota-Nau's knee blows up overnight or if he is unable to complete Friday's Pool C encounter.

Moore is on the bench after recovering from gastro but presumably one of the other front-rowers would have to move to hooker if he and Polota-Nau both went down.

Centre Anthony Faingaa all but confirmed his twin brother was no chance of playing.

"He's still in bed. The flu's knocked him around a bit," Faingaa said.

With five squad members in total not considered for selection this week, and skipper James Horwill resting his battered shoulder, the Wallabies have been at pains to play down their injury and illness troubles.

But it is hard to truly know the extent of Polota-Nau's apparent injury, if in fact there is any beyond wear and tear, because training sessions are held behind closed doors except for the first 15-minute warm-up period.

The Wallabies have also been confident this tournament that flanker David Pocock (back) and winger James O'Connor (hamstring) would be able to play in matches despite missing training sessions - and neither was able to back up.

The Wallabies will be on a hiding to nothing at the Cake Tin, with bookmakers handing the second-string US outfit a whopping 62.5 points start.

"Anyone that plays for the US, you look at any of their sporting sides, they play with passion," Alexander said.

"They've got a large population, such a big country, that any of the chosen few who get to represent their country do so with huge honour.

"So we're expecting the same physicality and ferocity they played Ireland with.

"They gave Ireland a good test in the wet, so we're expecting a big game from them."

Australia: Kurtley Beale, Adam Ashley-Cooper, Anthony Faingaa, Rob Horne, Drew Mitchell, Quade Cooper, Will Genia (capt), Wycliff Palu, Ben McCalman, Rocky Elsom, Nathan Sharpe, Rob Simmons, Ben Alexander, Tatafu Polota-Nau, James Slipper. Res: Stephen Moore, Sekope Kepu, Dan Vickerman, Radike Samo, Luke Burgess, Berrick Barnes, Pat McCabe.

United States: Blaine Scully, Colin Hawley, Tai Enosa, Junior Sifa, Kevin Swiryn, Nese Malifa, Tim Usasz (capt), JJ Gagiano, Pat Danahy, Inaki Basauri, Hayden Smith, Scott LaValla; Eric Fry, Phil Thiel, Shawn Pittman. Res: Brian McClenahan, Matekitonga Moeakiola, Louis Stanfill, Nic Johnson, Mike Petri, Roland Suniula, Chris Wyles.

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