"The early collisions will be very critical. We understand the Lions play a very physical brand of football - we've seen that throughout the games leading up to this Test," said Horwill.
"They have been very physical and confrontational and there is a big job for the boys up front to make sure we not only match that, but also ensure we take the ascendancy there.
"Paul O'Connell is a big guy in their team who has played a lot of rugby for Ireland and the Lions. I imagine he is someone they will turn to, and he is someone who's capabilities we are very wary of.
"He has been a great player for them throughout this tour and he has come back from his injuries a little bit rejuvenated and is playing some great rugby."
O'Connell and his experienced second row partner Alun Wyn Jones will go up against Horwill and Kane Douglas in one of the game's vital battlegrounds up front. The Qantas Wallabies may have an 80% success rate at the Suncorp Stadium, but the Lions have won all seven previous Tests in Brisbane.
"I don't know whether we are favourites or not or what the odds are, but I feel we have every chance to win. I think we are going to win," said Horwill.
"It's going to come down to making sure you work hard. In games of this magnitude there isn't a lot in it between players. Everyone across both teams is incredibly skilful, you have to keep your work rate going for the full 80 minutes because the margins are going to be very, very small.
"You have to get ready to take your opportunities when they come because they will be few and far between. It's a once in a career opportunity for the guys and this series is right up there.
"The World Cup is the pinnacle of our game, but a Lions series is a very close second. We are very aware of how big an opportunity this is."
The players of both teams awoke to find it was raining quite heavily in Brisbane on Friday morning, but Horwill says his side are not bothered by playing in wet weather. The Lions experienced the rain at Suncorp during their win over Queensland Reds two weeks ago and are well used to playing in the rain.
"I don't think the rain will affect us in any way. We had a week of it on the Sunshine Coast. We are used to training in it and by all reports the weather is going to clear later today," added Horwill.
"We are very excited to be playing at Suncorp, it's a great stadium. Then whole team really enjoys the whole atmosphere and the vibe that comes out of Suncorp and we've had great success there.
"We have played plenty of footie in the rain this year and our game is pretty adaptable. We are prepared for wet weather."