The 42-year-old assistant coach said that the team are ready for the final push after losing to the Wallabies by a single point in Melbourne last weekend.
"Seeing how we've trained all week, crikey we're ready for this battle," Rowntree told reporters. "We saw the reaction from Australia after they beat us last weekend - their captain James Horwill was crying.
"They threw everything at us and beat us by a point. We didn't get our game going and we know that. There's loads more to come from us and the guys are desperate to win. This is grand final rugby, the last throw of the dice with everything to play for.
"There's one huge performance left in us. These players are desperate for tomorrow. We have to go out and give it everything. This is the biggest game of our lives, as players and coaches. A lot of us have been involved in grand final games, but this is the biggest one for us."
The Lions have not won a tour series since defeating South Africa 16 years ago.