Following Australia's incredible tackling against Wales on Saturday, prop Sekope Kepu revealed where the staunch defence was born.
The Wallabies were camped on their own line for at least eight minutes during the crucial Pool A clash at Twickenham.
Michael Cheika's side were then reduced to 13 men after Will Genia and Dean Mumm received yellow cards in quick succession. However, Australia managed to keep out Wales with some dogged defence and went on to win 15-6.
Kepu revealed that the squad attended a training camp in Indiana in the USA, where they focused on tackling when fatigued. While standing in 38 degree Celsius heat, coach Michael Cheika ordered his charges to put on their full tracksuits for the next drill.
"There were days when we were in America that we still laugh about: 38 degrees and we had to throw on the suits, tops and bottoms," said the tighthead prop.
"It felt like 50 degrees in the suits, and we were just smashing each other for 30-40 minutes, and solving those problems.
"All the training has been about getting us fatigued and testing us under those moments when we are really buggered."
The defensive work the squad put in has so far paid dividends, as Australia have conceded just two tries in their four pool matches.
"The hard work has been done," added Kepu.
"When I was going through a bit of pain, you know you're hurting but you know the opposition is hurting as well if not worse.
"I just have full confidence in the work we have done but we have to keep building and not let it slip."
The win saw Australia finish top of Pool A – dubbed the "Pool of Death" in the build-up to the tournament – which secures them a quarter-final showdown with Scotland on Sunday.