RIO DE JANEIRO (AP) — Japan's stunning upset win over New Zealand and Sonny Bill Williams' subsequent withdrawal made for a rocky opening session for the return of men's rugby to the Olympics after 92 years.

Japan, which famously upset South Africa at the 15-a-side Rugby World Cup last year, produced an equally unexpected performance in the sevens format with the 14-12 victory Tuesday over 12-time world sevens series champion New Zealand.

"For most of us this is our last sevens series, so this is our only chance to prove to the world that Japan can beat any team," Japan back Lote Tuqiri said. "In sevens, it's anybody's game."

France kicked off the tournament with a surprising 31-14 win over error-prone Australia — the morning after a celebratory night for Australians after their women's team beat New Zealand to win the first rugby sevens gold medal at an Olympics.

Fiji, the favorite for the men's gold medal, had to wait until the last game of the first round to make its Olympic debut, and conceded the first try to host Brazil before taking control in the second half for a 40-12 win.

Argentina scored after the siren to edge the United States 17-14 in another tight tussle between closely-ranked teams in the earlier Pool A match.

Second-seeded South Africa was dominant in a 24-0 win over Spain, the last qualifier for the tournament, and Britain had a lopsided 31-7 win over Kenya.

Gold-medal contender New Zealand was expected to easily account for Japan, but went behind quickly when Teruya Goto dived over in the left corner and Lomano Lemeki converted from out wide.

New Zealand captain Scott Curry touched down under the posts, after Gillies Kaka made a strong burst up the right flank and floated a long pass in-field, and it was 7-7 at halftime.

Williams, the highest-profile player in the tournament after winning the 2011 and 2015 Rugby World Cup titles with the All Blacks, carried the ball into a tackle just after halftime and stayed on the ground, holding his left ankle. He was helped from the field and taken away on a motorized cart for treatment.

Akira Ioane powered through some poor defense to give New Zealand a 12-7 lead not long after but the usually clinical team in black couldn't close out, and conceded a late try to Komeli Soejima which Katsuyuki Sakai converted for the win.

As the Japan squad celebrated, the New Zealand team left the Deodoro Stadium without comment but later confirmed Williams had a partial rupture of his left Achilles tendon and could take no further part in the tournament.

The absence of Williams helped Japan.

"When Sonny Bill went off, it was a boost for us," Tuqiri said. "Our game-plan was to try and stop his offloading."

Lemeki compared the win in Rio with Japan's big victory at the 15s World Cup in England last year.

" You never see a minnow come here and beat a team which is supposed to be a gold-medal contender," he said. "I am still shocked, to be honest.

"As we played on, we started getting more confident and New Zealand started to struggle there a bit. We thought, if we keep on moving the ball around, the big guys will tire somewhere and eventually they did."

The second round of pool matches will be played later Tuesday, with the third round and quarterfinals set for Wednesday.