Under-pressure Australia know they can leave nothing back at the team hotel when they face what is expected to be an extremely physical US side in a must-win World Cup match on Friday. The Wallabies suffered a shock 15-6 defeat to Ireland last weekend and have left themselves in the position of having to win their two final pool matches, against the United States and then Russia, to guarantee a berth in the quarter-finals. They must now also do what no team has done before and come back from a pool game loss to lift their third Webb Ellis Cup. In the absence of rested lock James Horwill, scrum-half Will Genia will captain an Australian side showing six changes from the one that lost to Ireland. "We've got a big job ahead of us. The United States showed against Ireland that they are a totally committed opponent," said Genia. "Our own performance against Ireland, which was not up to the high standards we set for ourselves, will only have offered the Americans further encouragement." Genia added: "We just need to get out there and get it done, and that's pretty much the focus of the whole group. We're obviously disappointed with last week and want to go out there and make things right. "It was one poor game. We made poor decisions and the result was we didn't come away with the good result. "We worked on things to simplify the decision making." The US team come into Friday's game on the back of a narrow 13-6 victory over Russia, just four days after having gone down 22-10 to Ireland in a tough opening match. US coach Eddie O'Sullivan has made 14 changes to his team, key forwards Todd Clever and Mike MacDonald not even making the bench, but the former Ireland boss denied claims it was a second-string XV. "There's quite a few changes. It's a fairly mixed bag," acknowledged O'Sullivan. "But the guys are bringing lots of energy. A lot of the guys haven't suited up yet. "We're not throwing a bunch of rookies in there, there's quite a bit of experience running through the team. There are a lot of guys who were close to starting against Ireland or against Russia but didn't get the start." O'Sullivan admitted that his team were girded for a potential Wallaby backlash after their surprise loss to the Irish. "There's no question about it, any team that comes off a defeat will always want to make a statement and I have no doubt that will be the intent of Australia against us, to lay down a marker," he said. "We'll try to stick to our plans and system, and under presssure will look to not abandon ship. "Particularly in the last 20 minutes, (it's about) staying within striking distance as long as we can and staying on the horse as long as we can. "At times we've been a bit punchdrunk. We just need to hang in there and keep swinging and stay in the game until the last quarter. "That's always the plan when you're boxing out of your weight category."