USA defeated Canada 29-17 to remain unbeaten in the 2013 Nations Cup in Colorado, the only side to have won both of their opening two matches.
After the final round, which takes place on Wednesday 7 August, the top two teams will progress to the final which will decide the eventual champion, and all four teams are in with a chance after England earlier beat South Africa 18-17.
In the day's second match, Lynelle Kugler had given the hosts a 5-0 lead, which was doubled by Sevens star Nathalie Marchino on the eve of a landmark in the countdown to Olympic Games in 2016, with 5 August marking three years to go until Sevens' takes its place in Rio.
Canada reduced the deficit to five points when USA had a player in the sin bin, but replacement Sarah Chobot scored two tries either side of a Canadian try from Laura Russell to make it 22-12.
With less than ten minutes to go Jane Kirby gave Canada a chance of securing a second win in the competition as her try to took the score to 22-17, but the USA secured the win in the last few minutes with their fifth try of the match through Kristin Zdanczewicz, and now face England in round three on Wednesday.
"It’s been a really long time since we’ve beaten Canada,” Chobot said after the match. “I think it’s a big statement from where we’ve come from. If you look at Nations Cup two years ago, we lost to South Africa and beat them this year. This is a team that’s figured it out and it’s just a matter of getting better and better.
"England just had a tough series against New Zealand a week or so ago. So if we just do what we need to do – just take care of us – we’ll be fine.”
England fight back against SA
A decisive try from replacement Rochelle Clark in the last ten minutes ensured England won their first match of this year's Nations Cup.
Having trailed 17-7 at half time, England relied on two second half penalties from Lauren Cattell and Clark's try to ensure they won 18-17 and prevented South Africa from winning their first match of the Nations Cup, .
Zenay Jordaan had impressed for South Africa in the first half, scoring two of their three tries, as England were unable to break down their oppositions' defence.
Amy Wilson-Hardy scored England's only points of the first half, crossing for a try on her debut but after a chip and chase by South Africa in the 29th was gathered by England, the pace and pressure of the Springbok attack caused a turnover with Nolusindiso Booi taking advantage to give her team a 12-7 lead..
England now face USA on Wednesday, while South Africa meet Canada with all teams still mathematically in with a chance of reaching the final of the 2013 Nations Cup.
Women's Nations Cup fixtures and results
Tuesday, 30 July
Canada 29-25 England
USA 35-22 South Africa
Saturday, 3 August
England 18-17 South Africa
USA 29-17 Canada
Wednesday, 7 August
Canada v South Africa (16:00 local time, 22:00 GMT)
USA v England (18:00 local time, 00:00 GMT)
Saturday, 10 August
Third place play-off (16:00 local time, 22:00 GMT)
Final (18:15 local time, 00:15 GMT)
Matches will be streamed live at usarugby.org/tv