Never has it been more true that the big tests coming in the next month are going to be swung by the performances of the respective tight forwards.
But there's a twist to this truism as conditions in Japan are challenging tight fives in unexpected ways and damaging the impact of those teams whose athletes are built solely to thrive in the set-piece and collision.
This is a World Cup for the lean and mobile – not the heavy lifters who like to plod from breakdown to breakdown, throwing their weight around.
Japan is proving cruel and torturous for those sorts of big men. The heat, which across the country continues to hover around 30 degrees during the day, doesn't relent much at night.
The humidity varies but if it sits around 50 per cent, that seems like a small victory for the big men.
They'll take that in a heartbeat because often the humidity can be around 90 per cent and that's just about impossible for those with size to play the way they want to play.
Conditions are simply horrid for the tight five, who face a nearly impossible battle to give so much to the physical exchanges and yet still have enough to get around the park.
With the way things are, it feels like conditioning coaches have become all-important at this World Cup because technical expertise and strategy are not so much what matter as simply being able to go the distance.
We have seen enough now to be sure that the quarter-finals will be largely determined by which teams can keep their tight five charging for 80 minutes.
Fitness is going to be everything. This isn't going to be a World Cup determined by the power and set-piece efficiency of the respective packs – it is going to come down to which pack can keep running deep into the game.
It is a tournament set up for the aerobically capable – forwards who live just for the collision or just for the scrum, are going to be found out later in this tournament because if they are not fit enough, they will be able to do neither effectively.
The Irish pack were the first victim of the humidity. Against Japan, their vaunted tight five died 50 minutes into the game. Like a punch drunk heavyweight boxer their legs gave up first and the rest followed.
It was obvious they had nothing left in that last half-hour – a period in which a Japanese scrum conceding at least 5kg per man, shoved them clean off the ball.
In that last half hour, there were no Irish forwards coming on to the ball. No one wanted it. No one had the legs to go forward and so they held back, hoping someone else had the energy to cart the ball into contact.
It was a stunning collapse – their shape and structure disintegrating as the big men hid, knowing they were spent.
But while Ireland were the first serious contender to be exposed by conditions that are frighteningly oppressive for athletes carrying 120kg-plus, they won't be the last.
Others are inevitably going to be found out and the suspicion is it may be the northern hemisphere packs who wilt.
England have a big pack which has the power to smother every team on the planet when they are sucking in the fresh, cold air of a London winter.
If any team doesn't front physically against them at Twickenham, they can forget about winning.
England are too good at the set-piece and contact areas to be beaten if they are allowed to dominate possession and the pace of the game.
But the athletes have been built to play with that specific smothering style in mind, in conditions which allow their muscularity to flourish.
Their body shapes suggest they are gym fit. Strong, certainly, but genuine aerobic beasts? Maybe. But for 80 minutes in the heat and humidity?
It's the same with the Welsh, who bravely held on against Australia but were perhaps saved by the first-half lead they were able to build as the beneficiaries of some curious refereeing decisions.
Australia had more in the tank in the second half. They looked fitter, had more running in their legs and probably would have got over the top of the Welsh had they not been unfairly put so far behind.
If the first half had been played at a higher tempo, Wales may have been found out.
Not that any side should feel complacent as adapting to the aerobic challenges of Japan's sweltering conditions is proving difficult even for the All Blacks, whose forwards have long been conditioned to play a fast, skilled and mobile game.
During their training camp at Kashiwa before the tournament started, some of the tight forwards were losing around 5kg per training session even while excessively hydrating.
The fluid just pours out of the big men and no matter how hard they try to put it back in, the body can't re-set and 50 minutes of high-intensity football can feel like a lifetime.
The temperatures will drop through October and the humidity will recede but not by enough to stop being a factor or change the fact that the story of the World Cup, albeit with a twist, will still be shaped by the performance of the respective tight fives.