New Zealand, Fiji and England emerged as the unbeaten sides in the new cut-throat version of the World Rugby Sevens series in Hamilton on Saturday.

South Africa, who went into the tournament as co-leaders in the series with New Zealand, did slip up when they missed a conversion with seconds remaining against England and lost 21-19.

It meant England kept their unblemished record after they also beat Kenya 24-19 and have only Japan to play in their final Pool B match on Sunday while South Africa, who beat Japan 31-5, will play Kenya.

New Zealand, who opened their campaign with a 47-0 romp against Wales, finished the day with a 26-5 victory over the highly rated USA side in Pool A.

China upset a six-woman Fiji 17-12, but then fell to New Zealand on day one of the HSBC World Rugby Sevens series China upset a six-woman Fiji 17-12, but then fell to New Zealand on day one of the HSBC World Rugby Sevens series Photo: AFP / MICHAEL BRADLEY

Co-captain Tim Mikkelson said that with no quarter-final safety net they knew they could not afford to take the USA lightly.

"We circled this one. We said if we lose the game we won't be playing tomorrow, so a massive effort from the boys," he said.

New Zealand opened their campaign with a 47-0 romp against Wales, finished the day with a 26-5 victory over the highly rated USA side in Pool A New Zealand opened their campaign with a 47-0 romp against Wales, finished the day with a 26-5 victory over the highly rated USA side in Pool A Photo: AFP / MICHAEL BRADLEY

New Zealand's final pool game is against Scotland whose sole win was a narrow 24-19 victory over Wales.

Reigning series champions Fiji, whose best placing this season is a fourth in Cape Town, secured pole position in Pool D with a hard-earned 19-12 victory over Australia which put them on track for a likely semi-final showdown with New Zealand.

Pool C will boil down to who performs best when Canada play Spain and France face Ireland in their final matches before the play-offs.

Canada produced a surprise 26-21 win over Ireland and then battled to a 12-12 draw with France who had finished third in the Cape Town tournament six weeks ago.

In the women's competition, New Zealand emerged as the only unbeaten side in Pool A after China upset a six-woman Fiji 17-12 with Yang Feifei scoring the match decider right on full-time.

New Zealand, who face Fiji on Sunday, were untroubled on day one as they thrashed China 40-7 before producing an equally comprehensive 40-7 victory over England.

There are two unbeaten teams in the two other women's pools who are destined to meet early Sunday to determine who go through to the play-offs.

In Pool B the showdown is between Australia and the USA who are second and third in the standings after three rounds of the women's world series, while the match between France and Canada will decide who goes through from Pool C.