"The Waratahs were incredibly physical, and that's the key to any game," the Chiefs coach said. "The Brumbies failed to deal with that."
And when Rennie and co need an extra injection of physicality, there's one man to turn to.
Liam Messam has enjoyed another robust season in the defending champions' pack. Already the ironman of the competition coming into this campaign - having played more minutes than anyone across the last three seasons - the loose forward led his side with 15 starts and 1213 minutes.
But the true measure of Messam's value is far from his mere time spent on the pitch. When the Chiefs need go-forward ball, they turn to their co-captain. When they are trapped on their own line, grimly mounting an unwavering defensive resistance, Messam can be counted on to make a key hit.
The 30-year-old ranked first among his teammates in making 139 tackles this season, with Brodie Retallick (134) the only man within 20. And with ball in hand, Messam rated second in carries (behind only Tim Nanai-Williams) and third in metres made (behind Tom Marshall and Nanai-Williams).
He's the rare talent who can lead on both sides of the ball - repeatedly busting his way over the advantage line when the Chiefs are on attack and tackling himself to a standstill on defence.
So when the Chiefs attempt to set the physical tone tomorrow night, Messam's the player the Brumbies should fear the most.
"It's about making sure we go over to Australia and really keep that momentum, especially in the forward pack," said Messam, referring to the gains made during the hard-fought victories over the Hurricanes and Blues.
"I think the forwards have stood up and set that platform with their physicality. So it's just about making sure we get stuck into the Brumbies."
Messam ensured his charges did exactly that in last weekend's season-saving win over the Blues, leading the Chiefs with 19 tackles and inspiring a pair of impressive defensive rearguards. The first was in an opening 10 minutes spent camped on their own line, the next in the second spell while reduced to 14 men.
It was a typical captain's knock and that's before even mentioning Messam's impact on attack, illustrating both his dexterity and speed in nimbly picking off an Ihaia West pass and galloping across the open pastures of Eden Park to score a crucial opening try.
That triumph sealed the Chiefs' playoff place after living on the edge for a couple of games, and Messam hoped playing without a second life for two weeks running would aid the champs' cause in Canberra.
"The boys are very motivated - the last couple of weeks have shown that," he said. "We've been in the knockout stages since the Hurricanes game, and the boys have really responded the way, as a captain, I've wanted them to.
"Three weeks ago [the media] were writing us off, saying we couldn't do it, but we're still here."
Few would have given the Chiefs much of a chance of being in that position after the serious blow they suffered on their last trip to the Australian capital. The Brumbies' 41-23 destruction on Anzac Day was the nadir of an inconsistent campaign, but Messam believes his men have come some way since then.
"When we were over there in April we got a bit of a hiding and got a bit embarrassed. But there's different personnel - this team's grown since those disappointments.
"We've gone through a lot of hard times but I'm really confident that those hard times have made us stronger as a unit and as a family."
And, as the strongest member of a tenacious team, there is no prize given for guessing who sits at the head of that family.
The Big Chief.
Liam Messam in 2014
Chiefs ranks in parenthesis:
Starts: 15
(Tied-1st)
Minutes: 1213
(1st)
Tackles: 139
(1st)
Metres: 539
(3rd)
Carries: 143
(2nd)
Defenders beaten: 20
(5th)
