USA coach Eddie O'Sullivan reckons his side's 67-5 defeat to Australia was because they lacked the fitness to compete for the whole match.
The largely second-string Americans were gallant in a 22-5 first half performance, but the Wallabies eventually ran in 11 tries helped by their superior conditioning and slick handling skills.
"I was disappointed to lose," said O'Sullivan.
"Overall I thought the scoreline was a little bit harsh.
"Had we been able to retain more ball, and played more with the ball in hand, it would have helped.
"Holding on to the ball we looked pretty decent and we took Australia through a few collisions. Our lineouts were pretty much what we wanted them to be.
"I thought we played good rugby at times. We gave up some long-range tries which were a real killer."
O'Sullivan added that he thought his side, as is often the case with "tier two" nations, struggled to last the full 80 minutes of a highly physical encounter that left the Wallabies with a high injury count.
"We probably didn't start off as strong as we could and I thought at the end we got a bit tired again," the former Ireland boss said.
"A lot of inexperienced players were struggling with pace and the Australians didn't let up on us tonight."
US skipper Tim Usasz admitted his team had been handed a rugby lesson by the Wallabies.
"When you play the best sides in the world they really punish you for your mistakes, but that's we learned tonight," the Brisbane-born scrum-half said.
"We came out here to be measured against the best and we'll learn from it."
Usasz added that the US team had to seek more consistency and remain in the game until the last minute.
"It's about being more consistent," he said.
"When we play against the top teams, we get punished harder.
"I think we can mix it up with the top teams for periods of play, we just need to be able to do that for more periods of time.
"We know when we hold on to the ball we look really good."
Meanwhile, number eight J.J. Gagiano was left beaming after becoming the first player to score a try against Australia at this World Cup, deftly picking up from the base of a scrum and scampering past a despairing Rocky Elsom for a deserved try.
"It hasn't really sunk in yet," Gagiano said.
"The ball just came my way. I was not thinking about it, it just worked.
"It was just a great experience. It's not often you get to play a side like Australia. But you make one mistake and they come at you."