By Nick Mulvenney

RIO DE JANEIRO (Reuters) - Fiji sent New Zealand home, Japan continued their fairytale run to the semi-finals and Britain edged Argentina amid the high drama of extra time as the inaugural Olympic sevens tournament got down to the final four on Wednesday.

South Africa completed the quartet of nations who will play off for the first medals to be awarded in men's rugby for 92 years on Thursday after beating Australia 22-5 in the only quarter-final victory that could be described as comfortable.

World series champions Fiji's clash with New Zealand was an encounter that many thought would take place in the final until the All Blacks were stunned by Japan on Tuesday and beaten again by Britain in their final pool match on Wednesday.

Depleted by injuries to Sonny Bill Williams, Joe Webber and Scott Curry, the 12-times world series champions sneaked into the last eight when Fiji beat the United States in the last match of the opening round.

Fiji defied the wet conditions by opening the match with a magnificent move that took them from under their own posts to the New Zealand line in series of charges and handoffs, captain Osea Kolinisau touching down.

Gillies Kaka replied for New Zealand off a chip-and-chase while his team mate Rieko Ioane was in the sin-bin for a high tackle, but Jerry Tuwai skipped through the All Blacks defense to secure a 12-7 victory and set up a meeting with Japan.

"We've got one aim and that's to win gold medal," said Fiji's loquacious coach Ben Ryan.

"We're number one in the world, we're not trying to be arrogant, we're saying this is what we want. Silver will be as disappointing as bronze or fourth place."

Japan, ranked 10th out of the 12 teams that started the tournament, later added France to their list of victims on the remarkable run that has captured the imagination of the next Olympic host nation.

In a dramatic finish, the Japanese were trailing 7-5 with 16 seconds on the clock when Teruya Goto managed to force his way over the line.

"I can't explain it, we just train hard and somehow put it together on the pitch," said Japan's New Zealand born Lomano Lemeki.

"There's no pressure on us, the pressure's on the big teams, they are the ones supposed to be winning the medals, not us."

Even that finish was nothing compared to the drama that played out in the third quarter-final, which was only decided when Britain's Dan Bibby crashed through the Argentine defense four minutes into golden point extra time.

The match had remained deadlocked through a highly entertaining 14 minutes with Argentina skipper Gaston Revol failing in his attempt to win the match with a drop kick penalty goal in the dying seconds.

His Britain counterpart Tom Mitchell also failed with a similar attempt after three minutes of extra time but, less than a minute later, Bibby finally got the first points of the match to set up the meeting with South Africa.

(Reporting by Nick Mulvenney Editing by Alison Williams/Greg Stutchbury)