While the New Zealand All Blacks, South Africa's Springboks and Fiji look to cement their place on top of rugby sevens at the Olympic Games this July, they should be wary of the threat posed by the Stars and Stripes.

Known more for their strength in basketball and baseball, the United States could be a dark horse in the first rugby sevens contested in the Olympic Games this August.

In a media conference call yesterday, former English rugby union player Jason Robinson said of the fledgling American side: "They're a massive danger to the so-called powerhouses. I spent a day with the US team... they have the players to cause a few problems to the likes of Fiji and New Zealand."

Robinson, along with former Irish centre Brian O'Driscoll, are ambassadors for the HSBC Hong Kong Sevens, which kicks off tomorrow.

"To play sevens rugby you have to be extremely fit, I would have struggled to keep up with the demands of the game. It's nice to see these players in great condition," said Robinson, who has never represented his country in rugby sevens. He pointed out speedy American wingers Carlin Isles and Perry Baker as the ones to watch.

While the US are currently fifth in the overall standings, both Robinson and O'Driscoll agreed that the World Sevens Series is not a crystal ball to see who will stand on the podium in Rio de Janeiro.

The Irishman explained: "The quality teams always rise to the top, with the main teams Fiji, South Africa, Australia, New Zealand. But at the same time, you can talk about the US, and Argentina can do some great stuff. Any team can beat anyone - Kenya beat New Zealand (22-0) in Las Vegas in a pool match and nobody expected that."

It is this unpredictable nature of rugby sevens that appeals to O'Driscoll, whose country is not represented at the World Sevens Series.

He said: "That level of anticipation, not knowing what the game holds for you... also, you'll always have the support from the local fans - we had it in Vancouver when Canada beat Australia (14-12). That's the beauty of sevens, one-off performances you can't expect."

For Robinson, the return of the egg chasers at the Olympics has been a major boost to the sport.

He said: "We're seeing some of the underdogs like Russia and Portugal getting stronger. The exposure on the global stage which is not just great for the players, but also for the game as a whole.

"There are going to be millions of people introduced to rugby through the sevens...these are very exciting times for rugby."