Alonso, third behind Vettel's team-mate Mark Webber, said: "No surprises. They have the fastest package.
"But we put more pressure on them on Sundays. We will take our opportunities and fight every lap in the race."
Alonso was only 0.008secs slower than Webber in terms of outright pace and his fastest lap during his race-simulation run on heavy fuel was only 0.161secs slower than Vettel, who was 0.18secs behind pace-setter Lewis Hamilton.
Red Bull duo praise Austin circuit
Alonso said: "[Red Bull] are first and second in all the practice [sessions] and in qualifying, and if everything goes normal first and second in the race - but in the race sometimes everything is not going normal.
"We know their strong point is the speed of the package and our strong point is the united team, we try to use our strong points.
"The important thing is to score 11 more points than them in these [final] two races. We are still convinced we can do it.
"But we are not surprised when they are first and second in practice and qualifying."
"Starting third would be fantastic but we were third many Fridays and the McLarens, Lotus they seem to use the potential that on Friday maybe they hide.
Not impossible to win - Hamilton
"So it is possible we have a tough qualifying, but we were seventh and ninth in Abu Dhabi and we scored three points more than Vettel, and if we are better than seventh I think we can score even more points than him."
Asked if he was worried about it, Button said: "No because I don't think many people got a good lap in on low fuel. But the Red Bull's a quick car and Seb's very quick in it so they are going to be the people to beat in qualifying, but the gap isn't as big as it looks I don't think.
"They look very fast and it's not just their outright speed it's how easy they can get the tyres working.
"They can do a very quick lap on lap one and for us it's impossible, it's just the way the car works but I don't think there is going to be a second gap between us and Seb in qualifying. I think we'll be a lot closer."
Vettel, who lost half the second practice session while his mechanics fixed a water leak, also said little should be read into his advantage because of the newness of the circuit.
"Friday you need to be careful [of the times]," he said. "The fact that it was very slippery can make a bigger difference than you think.
"There were one or two surprises in terms of pace. It is important we are amongst the guys at the top and we [will] try to improve the car for tomorrow to make sure we stay there."
United States Grand Prix 2012, day two
Saturday, 17 November: Third practice 15:00 GMT; audio and live text online. Qualifying 17:55 GMT; 5 live sports extra and live text commentary online. Qualifying highlights; 21:00 GMT, BBC Two
United States Grand Prix 2012, day three
Sunday, 18 November: Race 18:55 GMT; 5 live and live text commentary online. Race highlights; 22:25 GMT BBC One
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