Japan stunned New Zealand 14-12 in their opening match of the Olympic rugby sevens on Tuesday as All Blacks star Sonny Bill Williams came off injured.

Williams was helped off the field before a medicab assisted him after full-time with fellow All Black, and shadow player Liam Messam seen entering the New Zealand dressing rooms afterwards.

The opening pool match upset was an echo of Japan's famous victory over South Africa in the 15-a-side World Cup in Brighton, the biggest shock the sport has ever seen.

"It's unbelievable, you never see a minnow team come here and beat a team supposed to be a gold medal contender. I’m still shocked, to be honest,” said Japan's New Zealand-born playmaker Lomano Lemeki.

This is how to celebrate after defeating #NZL in rugby! #JPN rugby players celebrate after their upset! #Rugby7s pic.twitter.com/lnMNsIggN5 — Rio 2016 (@Rio2016_en) August 9, 2016

At a sun-kissed Deodoro Stadium, with massive support for the underdogs, Truya Goto squeezed past Rieko Ioane's tackle to dot down in the corner to open Japan's account.

Their lead was short-lived, however, as Gillies Kaka exchanged passses with Akira Ioane before putting in skipper Scott Curry.

Akira Ioane crossed for New Zealand's second but Kameli Soejima scrambled over to draw the scores level, Katsuyuki Sakai's conversion pulling the Japanese two points clear for a famous win.

"As we played on, the boys started to get more confident," said Lemeki.

"New Zealand started to struggle a bit there so we thought if we keep moving the ball around the big guys would tire somewhere and they did."

Lemeki added: "We've just shown that any team can win in sevens.

"We had a gameplan against New Zealand: if we get penalties kick long, slow the ball down and play at our pace."

Asked if he thought the Kiwis had taken the Japanese too lightly, Lemeki said: "I think they might have, but I don't think they'll be doing that again!"

Soejima, who scored Japan's second try, was left revelling in the fun of sevens.

Japan have done it again! This time their 7's team beat NZ! Amazing performance and result #Olympics2016 — Will Carling (@willcarling) August 9, 2016

"Our main secret is to come out and enjoy the sevens, it's the Olympics, it's the highest level of any sports, so give it your all," the Fiji-born flyer said.

"Most of all it's our last event of the season after the Sevens Series, and the last chance for everyone to prove to the world that Japan rugby can beat any team.

"Give it your all, give it your best shot, most of all play our Japan way.

"It's sevens, it's anyone's games!"

Seeing Williams, a two-time World Cup winner with the All Blacks, being helped off was a "boost", Soejima said. Williams hobbled off in the second half for treatment to his left leg.

"He's like a main weapon in the New Zealand team so that was our gameplan, just try to stop him, especially in offloading."

Lemeki added that it was not all about Williams.

"Sonny Bill's a good player, no doubt. But we look at every player as a good player on the New Zealand team,” he said.

"I think the Japanese boys started to smile a little bit because they wouldn't have to tackle him!"