Rugby giant-slayers Japan have pulled off another amazing big-stage boilover - beating the All Black Sevens 14-12 at the Rio Olympics.

LISTEN ABOVE: Newstalk ZB Olympics reporter Jack Tame speaks with Andrew Dickens

Just like last year's astonishing Rugby World Cup upset over South Africa, David upset Goliath in an opening pool match which had the Deodoro Stadium crowd in raptures.

"It was unbelievable," said Japan back Lomano Lemeki. "You never think you would see a minnow team come here and beat a gold medal contender.

"I'm still shocked to be honest."

The major drama did not end there with New Zealand's highest-profile Olympian, Sonny Bill Williams being ruled out for the rest of the tournament.

Williams partially ruptured his Achilles tendon after he took a crash ball into two defenders early in the second half.

Joe Webber, won't play again after being medi-cabbed off late in the game with a shoulder injury.

Captain Scott Curry said they were guilty of making too may mistakes.

"Without the ball it's pretty hard to score and when we got it we knocked it on or turned it over at the ruck."

The All Black Sevens let themselves down with poor handling while the underdog Japanese skilfully controlled the majority of the game by slowing it down to their pace and not allowing the second seeds get into rhythm.

Playing well above themselves, the underdogs scored the opening try but the Kiwi side led 12-7 with two minutes to go before Kameli Soejima crossed out wide and Ktsuyuki Sakai nailed a clutch conversion.

NZ almost stole the match at the death with a 90m raid after the siren but heroic defence ultimately ensured another fairytale result.

"That was crazy," Lemeki said. "Everyone was flying just like a kamikaze plane trying to chop their legs off - as long as we didn't let them score a try, if we got knocked out that's fine."

Lemeki felt their dejected opponents took his side lightly, while the result meant the world No.15-ranked side were now serious medal contenders.

"They are going to think they're Superman," he said of his teammates.

It was one of three upsets on captivating day one of the men's sevens with Australia falling 31-14 to France and Argentina also surprising USA 17-14.

"It means any team can win on any day in sevens," Lemeki said. "It's wide open."