Jill Wittner gave the US a lead, much against the run of play and off a fortuitously-awarded penalty corner, deflecting a drive over keeper Grace O'Hanlon.
Midfielder Ella Gunson equalised, niftily deflecting a strong Olivia Merry drive high into the US net before halftime.
However Taylor West scored twice in the second half, once with a penalty stroke after Frances Davies bundled an attacker over, and once with a reverse stick shot which bobbled twice before crossing the line.
In the early game, the world No 1 team the Netherlands easily completed a 100 percent record in pool play with a 3-0 win over South Korea.
The Dutch had 25 circle entries to three by the world No 9 Koreans and a 19-1 shot advantage.
The goals came from Margot van Geffen from a penalty corner just before halftime, the in-form Maartje Krekelaar in the 41st minute and another penalty corner strike from Caia van Maasakker.
"It's perfect and we're happy," van Maasakker said of their pool play form.
So a day off today before the quarter-finals perhaps?
"No I think we will go training. I like to train."
In tomorrow's final pool B matches, Germany play Argentina, both in good form with two-from-two records, and England face China. There are two quarer-finals on each of Wednesday and Thursday.