The Black Ferns stunned the Olympic champions with a comfortable 28-5 victory in the final as they ran in four tries, with Portia Woodman, Tyla Nathan-Wong, Ruby Tui and Alena Saili all getting on the scoresheet at the Sam Boyd Stadium.

The result for the New Zealand side saw them partially avenge their loss at the hands of Australia in the first-ever women's Olympic rugby sevens gold medal match in Rio de Janeiro last year.

New Zealand now lead the race for the overall crown on a total of 56, with Australia six points adrift in second place and Canada a further four behind.

The Canadians silenced a packed home crowd inside the arena as they overcame hosts United States 31-7 in the bronze medal match.

"We came second in the Olympics to Australia so to beat them now in two finals this season, in Dubai and again tonight, is kind of bittersweet," said Woodman. 

"But we are delighted with this win, we're going to enjoy it and then look ahead to the next tournament and see how we go from there."

New Zealand reached the showpiece contest at the event in Las Vegas with a narrow 12-7 win against the US, while Australia emerged on the right side of a high-scoring encounter with Canada as they claimed a thrilling 26-17 triumph.

Spain won the Challenge Trophy by ousting England 10-0.

In the men's event, the US produced a superb comeback as they rallied from 19-0 down to beat Argentina in a pulsating quarter-final.

The South American nation opened up a commanding lead but the US, spurred on by the raucous backing of the majority of the fans inside the stadium, refused to accept defeat and eventually clinched a dramatic 21-19 win.

The hosts remain in contention for their first-ever World Rugby Sevens Series title on home soil and will go up against current overall leaders South Africa, who beat Australia 17-14 in their quarter-final, in the last four.

"I think we showed that we can be down but we're never out," said USA captain Madison Hughes. 

"We are going to have to play some great teams during a tournament but we just have to stick together and keep fighting."

The other semi-final will be between Olympic gold medallists Fiji, who swept aside Canada 31-12, and New Zealand who outclassed England with a 19-0 victory.

Both matches are due to take place on the final day of the Las Vegas tournament tomorrow.