Wales and Australia wins over England in the first two matches helped both into the quarter-finals but the losers of the third seismic clash must take on South Africa with a potential semi-final against New Zealand to follow.
The winners of the match-up between the losing semi-finalists from four years ago will top Pool A and embark on a gentler route, probably starting with Scotland.
A draw would send Australia through as pool winners because of their superior points differential but all such timid calculations do not sit well with the confident Wallabies coach.
"We want to win every game, no team ever won the World Cup losing a game," Michael Cheika told reporters on Thursday.
"I don't subscribe to the theory that if you win your group, you are going to get an easier run. That is disrespect the opposition and that is not what we are about.
"It is going to be a big game for us. It is going to be a massive battle and pretty painful."
Australia have won their last 10 matches against Wales but all but three have been by seven points or less and the Welsh, not to mention their coach Warren Gatland, were pivotal in the British and Irish Lions triumph Down Under two years ago.
A battle at the breakdown for the ages can be expected with both sides fielding two specialist openside flankers -- David Pocock and Sean McMahon for Australia with Sam Warburton and Justin Tipuric lining up for Wales.
HIGHLY REGARDED
McMahon gets his chance to show why he is so highly regarded after Michael Hooper was banned for one week for a shoulder barge in the 33-13 victory over England.
An injury to Rob Horne gives winger Drew Mitchell the opportunity to add to his tally of 12 World Cup tries in an experienced backline boasting 421 caps.
Gatland, whose job has been made harder by a succession of injuries to key players, made six changes to his side from the team that beat Fiji last week and has gambled on playing three backs out of their regular positions.
George North, whose battles with Israel Folau were one of the highlights of the Lions series, moves in from the wing to centre, Liam Williams replaces him out wide and Gareth Anscombe, usually a flyhalf, makes his first start for Wales at fullback.
"They are favourites. They played extremely well against England, the best a team has played alongside the All Blacks," Warburton said of the Wallabies.
"It doesn't really bother me. It's anyone's game. Favourites, underdogs and odds are irrelevant."
Wales: 1-Paul James, 2-Scott Baldwin, 3-Samson Lee, 4-Luke Charteris, 5-Alun Wyn Jones, 6-Sam Warburton (captain), 7-Justin Tipuric, 8-Toby Faletau; 9-Gareth Davies, 10-Dan Biggar, 11-Liam Williams, 12-Jamie Roberts, 13-George North, 14-Alex Cuthbert, 15-Gareth Anscombe
Replacements: 16-Ken Owens, 17-Aaron Jarvis, 18-Tom Francis, 19-Jake Ball, 20-Ross Moriarty, 21-Lloyd Williams, 22-Rhys Priestland, 23-James Hook
Australia: Team: 1-Scott Sio, 2-Stephen Moore (captain), 3-Sekope Kepu, 4-Kane Douglas, 5-Dean Mumm, 6-Scott Fardy, 7-Sean McMahon, 8-David Pocock, 9-Will Genia, 10-Bernard Foley, 11-Drew Mitchell, 12-Matt Giteau, 13-Tevita Kuridrani, 14-Adam Ashley-Cooper, 15-Israel Folau
Replacements: 16-Tatafu Polota-Nau, 17-James Slipper, 18-Greg Holmes, 19-Rob Simmons, 20-Ben McCalman, 21-Nick Phipps, 22-Matt Toomua, 23-Kurtley Beale
Referee: Craig Joubert (South Africa)
(Reporting by Nick Mulvenney, Editing by Ed Osmond)
