RIO DE JANEIRO -- This was the most bonkers, eventful, turbulent day of sevens we have seen in recent memory. The formbook wasnt just ripped up, it was shredded. At the forefront was the gargantuan shock from Japan as they defeated New Zealand, a match which saw superstar Sonny Bill Williams pick up an Olympics-ending injury. Here we round up the five key talking points from a momentous day one at the sevens.The giant killers strike againIn 2015 the Brave Blossoms, Japans 15 team then managed by Eddie Jones, turned Brighton into a sea of red and white after they caused arguably the biggest upset in the history of professional rugby when they beat the Springboks. Just under a year on and theyve done it again as they clipped the Kiwis wings, seeing them off 14-12.Like that memorable afternoon last year, this was a performance packed with heart and an ability to turn possession into points. For a team who finished 15th in the World Rugby Sevens Series, this was a remarkable result, much to the delirium of their support in the crowd. It bodes well ahead of Tokyo 2020.It is unbelievable, Japans Lomano Lemeki said moments after their win.You never see a minnow come here and beat a team which is supposed to be a gold medal contender. I am still shocked, to be honest.But having lost their second match to Great Britain -- they had a kick to draw the match with the final play -- they now need a win over Kenya on day two to stay in the mix for a spot in the quarterfinals.The wounded Kiwi is a dangerous beastAfter the womens tournament went to script with the two best sides New Zealand and Australia competing in the final, the Black Ferns male counterparts wouldve fancied their chances of going one better and claiming gold. But if they are to take the top honour here in Rio, they will have to pluck the most majestic New Zealand White rabbit out of the hat.The Japan result was the shock of day one and with Sonny Bill Williams and Joe Webber sidelined, they have lost two of their key match-winners; they are players who can turn a game on its head. But they bounced back impressively against Kenya, winning 28-5, and in front of the watching Richie McCaw they did their best to emulate his warrior spirit as they kept themselves in the tournament. Injuries can galvanise a team; underestimate the Kiwis at your peril and that win over Kenya will give them heart.Tom Mitchell pulling the strings for Team GBIt was two from two for Great Britain and the perfect start for Great Britain on day one of the sevens. Tom Mitchell, their captain, was the fulcrum around which the rest of the team pivoted. He played brilliantly as sweeper. With players like Dan Norton and Dan Bibby in their ranks, they have brilliant finishers and while their story may fly under the radar amid the events of Japans game against New Zealand, they are ticking along nicely.At the end of the day, you have to come away with the results, Mitchell said after their 21-19 win over Japan. We always go out to deliver good performances and while Japan performance wasnt as a good as wed have liked, well take the two wins, rest up tonight and go into tomorrow looking to deliver better performances and get the wins.Fiji lead the way but South Africa are going wellWith the their prime minister watching on from the stands - he sat with spare seats either side, flanked by security guards - Fiji were the most consistent side on day one. They survived the scare of playing hosts Brazil in the opener and although their lineout faltered, their cutting edge was sharper than any of the other teams. Argentina gave them a good challenge but again their class shone through.But South Africa also impressed. They eased past Spain and then dispatched the impressive France outfit in their second match. Elsewhere Argentina and Australia are still in the mix after going one from two but it is Ben Ryans Islanders who are leading the charge for gold.U.S. open with defeat but get campaign back on trackThe USA sevens side would have been left kicking themselves after their opening round defeat to Argentina. With the clock ticking towards fulltime, they were winning 14-12 only for Matias Moroni to crash over in the corner and steal the match. And all that after Argentina were reduced to five men with two sin-bins in the second-half. They lacked precision and their x-factor runner Perry Baker never got a chance to stretch his legs.But against Brazil they managed to get the key 26-0 win under the belt with a performance that saw Nate Ebner score a try and also get sin-binned. 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