The Aussies played a near-perfect half of rugby to go into the break 19-0 up but had to overcome a serious bout of second-half wobbles and shut down a late raid by the US, prevailing 26-19 at Allianz Stadium.

"The first seven minutes was probably some of the best football we've ever played and that's where possession is everything," coach Andy Friend said.

"We probably fell asleep a bit there but the pleasing thing is we came back and managed to score at the end. We needed to win that lineout and we didn't, but we managed to shut them down at the end."

The win was an important pre-emptive strike ahead of remaining pool games against Canada and Scotland on Saturday, and a solid shot of confidence for a young team.

"It's a massive one, they're a tough side, USA, and we've struggled against them in the past," Friend said.

Scotland smashed Canada 52-5 in the final match of the day, making them the team for Australia to beat on Sunday.

Australia were rampant in the first half, feasting on turnover ball, intercepts and a yellow card against America to score three tries in the first six minutes.

Sydney boy Boyd Killingworth opened the scoring and Ben O'Donnell was next, notching his 10th world series try after the US went down a man. John Porch scored next for Australia putting the home side up 19-0 at the break.

The US struck back hard and fast to narrow the gap to five points just three minutes in and draw level a moment later.

The man they call 'Speed Stick', Perry Baker, wrought havoc, dancing around two defenders to score and make break after break to test the defence. Martin Isefo scored next and a Baker break helped put Ben Pinkleman over in the 11th minute.

Youngster Tim Anstee put Australia ahead again with a minute left, making it 26-19 after the conversion, but the match looked headed for a draw when America stole an Australian line out and track star Carlin Isles took the ball to the outside.

Veteran Jesse Parahi, returning from a stint on the sidelines, made his contribution count off the bench, wrapping up Isles to close out the match.

The US were the surprise packages of last year's world series, finishing fourth - one spot above Australia - in a season that saw flyer Baker top the try-scoring stats and earn the title of world men's sevens player of the year. The Americans have struggled this season, finishing sixth in the first leg at Cape Town and going winless in Dubai.

Australia are without young gun Simon Kennewell, who injured his knee in training this week. Considered one of the squad's best players, if not the best, Kennewell's sudden absence threatens to derail the side's plan to take home the cup this weekend.

Earlier in the day, Olympic champions Fiji demolished Samoa five tries to one and reigning world series champions South Africa blitzed Papua New Guinea eight tries to none.