Rugby World Cup: Teams low on gas after brutal pool phase

Losing Peter O'Mahoney to injury is a blow to Ireland's World Cup hopes. Photo / Brett Phibbs

Fatigued sides may struggle in knockout stage after pushing themselves to limit.

It's possible that some people are seeing a different World Cup - one that hasn't factored in the physical and emotional toil that some teams have already endured.

The well is only so deep in this business - players can only keep giving for a set time and typically it's not five weeks. It's already an enormous task to front three times in three weeks to survive the knockout rounds.

But some teams that have made it to the last eight - Wales, Australia, Ireland, France and Scotland - have had to give plenty just to make it this far. Maybe too much.

Australia produced one of the most impressive and brave defensive efforts seen at a World Cup when they were down to 13 men. When Dean Mumm was given his yellow card - following Will Genia to the sidelines - the game was there for Wales. They knew it, the Wallabies knew it, everyone in the world knew it. It was a huge 10 minutes because the prize wasn't just victory - it was a quarter-final against Scotland. For the loser, South Africa awaited. Both teams were desperate, both teams gave everything.

But Wales couldn't score because for 10 minutes they had to play against heroic resistance. By the end, Wales were drained - as much by what they had let slip as by what they had put in.

The Wallabies were too tired to be elated and nervous, too, having seen Israel Folau and David Pocock come off with injuries that may yet not come right.

The view in most places was that the Wallabies confirmed their status as the new favourites and Wales a team best avoided in the last eight.

There was an alternative way to look at it, though: that Australia and Wales have had to play too much rugby too early in this competition.

Wales have had awful luck with injuries. They lost Liam Williams against Australia and they are being forced to call up men they didn't think they would have to.

Still, if they believe in themselves and play wider and faster, they could beat South Africa at Twickenham. If they do that, however, they will be just about out on their feet for the semi. They will have something left - just not enough to mount the sort of challenge they will want to.

Australia have unquestionably been impressive and look to have fixed all parts of their game. They can't keep this sort of intensity up for five consecutive weeks, though.

At some stage, and it could well be the semifinal, their energy levels are going to be dangerously low. They have given so much it would be miraculous if they are still firing like this in the semifinal. Fatigue is going to get them.

Ireland are going to do well to pick themselves up after a huge effort to beat France. They lost Johnny Sexton - the one player everyone said they couldn't afford to lose; their captain Paul O'Connell and their seriously good flanker Peter O'Mahoney to what looked like significant injuries in all three cases.

Argentina will fancy they are catching them at a vulnerable time.

The teams that have shone brightest so far have put the most in, which, by definition, means they are the teams that have had the most taken out of them.