On Saturday morning, the men’s and women’s US Eagles will kick off at the Nacra Sevens tournament in Cary, North Carolina. By Sunday evening, both teams will know if they have qualified for rugby’s return to the Olympic Games, in Rio de Janeiro next year.

The winners at WakeMed Soccer Park will take automatic places in Brazil; the losers will look to a final repechage next year.

For the American men, in a pool full not so much of rugby minnows as plankton – Mexico, St Vincent, Jamaica and Barbados – that will most likely mean beating the other big beast, Canada, in the final. For the women, with the Canadians already qualified from a World Series in which the US lost a Rio spot to England on results count-back, it means whaling on teams they should expect to swallow in a gulp.

Accordingly, women’s coach Ric Suggitt has rested key players, including the dangerous Victoria Folayan, that they might be fresh for this summer’s Pan-American Games in Toronto.

“I’m always confident,” he told the Guardian last week, asked about his approach to the Olympic qualifying event. “I just have to figure out a way to get our players to feel like I do.”

Suggitt added: “We’re not underestimating anyone – I lived in Trinidad for a while and I know what island rugby is like. But with the team and the players we have now, we should be OK going into Nacra.”

In comparison, for Mike Friday, the English coach who took over the US men this season, the Nacra (North America and Caribbean Rugby Association) event will be “a funny one, a bit surreal, really”.

A normal game of sevens lasts 14 minutes, seven each way. Finals last 20. In Cary, so much will rest on so little.

Friday continued: “You look at some of the other qualifiers and the European qualifier is over three legs. What concerns me is that I want us to be talking about a team having earned the right to go [to Rio]. I don’t want it to be down to some funny old calls in one game, or the refereeing maybe not being the standard it might be.”

The feeling remains, however, that it may take unfriendly calls, balls and squalls to knock Friday’s team off their game. They finished sixth on the World Series ladder to Canada’s ninth – the top four, Fiji, South Africa, New Zealand and England, qualified for Rio outright – and took a first tournament win at Twickenham in London in May. The squad has pace (in football converts Carlin Isles and Perry Baker), power (Danny Barrett, Garrett Bender), world all-star team experience (Zack Test) and considerable guile (captain Madison Hughes).

And yet, and yet. In a season that Friday said ruefully had “a little bit of what-if”, after a slow start in Dubai and South Africa that eventually counted against automatic qualification, the Eagles generally had a hold on Canada, winning three of four meetings. But in Glasgow, the event before London, the tables were turned: 40-0.

“We’ve shown how fragile life can be if we choose to be slightly off,” Friday said of that defeat. “I say we ‘choose’, it wasn’t a conscious decision. But it showed we need to be right with our concentration, our work ethic, our commitment to one another. If we aren’t, sevens can be brutal. And that’s exactly what happened in that game in Glasgow.”

For Nacra, Canada have named a squad boosted by European professionals, the Ospreys’ Tyler Ardron and Jeff Hassler, and the return of long-injured captain Phil Mack. Friday’s squad includes the experienced Saracens back Chris Wyles, but otherwise sticks to the players who came on strong in the series this year.

USA win the London Sevens, at Twickenham

Danny Barrett has had a breakout year on the world sevens circuit. Photograph: Christopher Lee/Getty Images

“A one-off game of sevens always can be a flip of a coin,” Friday said. “You can get the first kick-off slightly wrong, you can be down to six men. You can make two mistakes, you can be down 21-0. There’s so many ifs and buts. All we can do is focus on controlling what we can control.”

Ever since the US last tried invading Canada, during the war of 1812, that has seemed a sensible policy. Asked if the best Nacra result for rugby in north America would be the Eagles through to Rio and Canada to follow through the repechage, Friday laughed.

“Yeah… I mean, for us it would be. If we can get the job done it will complete a massive year for US rugby, a massive transformation from being relegation candidates from the series to Olympic qualification.

“To a certain extent, the world is fearing the USA sevens team right now. Nobody wants to play us.”

That much is true – and by winning so impressively in London, the US showed that a gold medal in Rio, and thus the successful defence of an Olympic title won in Paris 91 years ago, is not remotely out of the question.

But rare is the Canadian who doesn’t fancy bringing an American down a peg or two. Barring the mother of all upsets from Barbados or the Bahamas, Sunday night’s men’s Nacra final will pit two serious sevens teams against each other for a seriously significant prize. It should be 20 minutes of truly magnificent sevens.