The Pool C clash falls on the same day as the 10th anniversary of September 11 and Best believes Eddie O'Sullivan's side will feed off the emotion of the occasion.
"September 11 will be a momentous day for them. They'll be charged," said the Ulster hooker. "The first 10 minutes will reflect that and we must make sure that at worst we're able to match their physical intensity."
Ireland's workload since their arrival in Queenstown last Thursday has been light with players enjoying a number of activities such as bungee jumping and boat rides as they recover from jet lag.
But the intensity in training has now been ramped up as they prepare for their move to New Plymouth, where they will meet the USA.
"The mood in the camp is very good. The setting we've been in since our arrival has been relaxing," said Best. "Everyone has enjoined themselves. Initially we had a few light training sessions to get rid of the jet lag.
"Today this was the start of the preparation week and training went very well. Everyone was clued in.
"Today you could feel there was a big step up in intensity. It's game week. This is why we were here.
"We've enjoyed ourselves over the last few days but the bottom line is we have a game on Sunday that we must go out and win.
"Now we know that it's game time and we're tuned into that."
The draw has been generous to Ireland with the USA offering a kind introduction to the World Cup before the key fixture against Australia arrives six days later.
Best, however, insists the minnows will not be underestimated.
"We've done a bit of analysis on the USA and as the week goes on we'll start to intensify that," he said.
"We've got quite a bit on them from World Cup and Churchill Cup games. We'll give them the respect they deserve."
There was no change on the injury front with Cian Healy and Gordon D'Arcy, who arrived in New Zealand only yesterday, playing a limited part in training.
Healy (eye) is a doubt to face the USA but D'Arcy (calf) is on course to make Ireland's tournament opener.
Sean O'Brien (knee), Rob Kearney (groin) and Brian O'Driscoll (shoulder) trained fully.
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