Cooper, who was born in New Zealand, has been subjected to a barrage of terrace abuse through Australia's campaign due to a long-running spat with the All Blacks captain Richie McCaw.
And although the 23-year-old was disappointing in last weekend's quarter-final victory over South Africa, Horwill has predicted a performance to remember from Cooper.
"I think he will have the best game he will ever have. He has just been training well and he is looking forward to it," said Horwill.
"Everyone seems keen to talk about one person, but it is about the group going out there and getting the job done.
"We understand the magnitude of the game at hand but everybody is pretty relaxed and I think that is a good sign.
"We are confident that we understand what we need to do to get the result."
The Wallabies will be without full-back Kurtley Beale, who has been ruled out with a hamstring strain.
Beale was initially named in the side with coach Robbie Deans determined to give Australia's player of the year every chance to prove his fitness.
But the 22-year-old was withdrawn prior to today's captain's run at North Harbour Stadium, in which he was only able to play a limited part.
As a result, Adam Ashley-Cooper will switch to full-back with Anthony Fainga'a to start at outside centre. Rob Horne comes onto the bench.
"It is disappointing for Kurtley himself. He worked very hard to do everything that he could to get right," said Horwill.
Beale is confident he will be fit to play next week in what he hopes will be the Wallabies' fourth World Cup final.
Australia will have to beat the All Blacks at their spiritual home of Eden Park for the first time since 1986 - but making history is not what drives them.
"All the inspiration you need is the fact we win and you are in the World Cup final," said forward David Pocock.
"You grow up playing rugby dreaming about playing in a World Cup final. You do not need other inspiration and if you do you should not be here."
Pocock's clash with All Blacks captain Richie McCaw, who has been struggling with a foot injury, will be pivotal.
"There are a number of good sevens in the tournament and it is a big challenge. Whoever wins goes a long way in deciding who is better at number seven on the night," said Pocock.
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