South Africa, who qualified for the Games by winning the final repechage slot last month, went ahead with a try by Selwyn Davies, who then delivered a great kick and chase which he scooped up to gift Tristan Leyds a second try, both converted by Leyds.
Tokyo silver medallists New Zealand closed the gap to 14-7 at halftime through Moses Leo and then hammered at the door throughout the second half but were kept at bay by some incredible South African defence.
The clash between Argentina, who won the regular season world series, and France, who beat them in the new end-of-season grand final, was always likely to be high-octane.
Fiji proud
But there was an extra edge at the Stade de France with the Argentines being booed by French fans in the wake of a video showing some Argentine international footballers singing a racist song about the French team.
Those jeers soon turned to cheers as France roared into a 21-0 halftime lead with a try by Andy Timo and two for Aaron Grandidier Nkanang, the second a spectacular diving one-handed finish.
Argentina hit back through Rodrigo Isgro and Marcos Moneta to set up a tense finale but the stadium exploded when replacement Antoine Dupont burst clear to settle it with France's fourth try in the final play.
Fiji extended their proud record of winning every match they have played at the Olympics since sevens was introduced in 2016 but they were pushed all the way by a remarkable Irish side who are relative newcomers to the format.
Opportunist try
Iosefo Baleiwairiki claimed the first Fiji try after 30 seconds but Ireland led 10-7 at halftime after two well-finished tries by Chay Mullins.
Zac Ward then squeezed in for a third Irish try and a huge shock looked on the cards, but a typical length-of-the field score finished by Joji Nasova and an opportunist try by Waisea Nacuqu after Ireland failed to gather the restart suddenly had Fiji four points ahead and they then defended superbly to hang on.
Australia, beaten at the quarter-final stage in the last two Olympics, were always on top against the United States and led 10-0 at the break after tries by James Turner and Corey Toole.
Maurice Longbottom then took centre stage, kicking a rare drop-goal penalty then finishing the game with a try to complete a dominant 18-0 victory.
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