Abby Wambach had given the U.S. a deserved first-half lead, but despite their superiority, Pia Sundhage's side were unable to finish off their opponents until late ont. Alex Morgan spurned a series if opportunities before Leroux came on to end New Zealand's resistance three minutes from time.
Despite their underdog status, New Zealand showed little fear as they played with positivity from the off. Keeping a high defensive line may have been admirable, but it also invited trouble, as the U.S. and the pacey Alex Morgan in particular looked to get in-behind time and again.
It was that play that really should have led to the opening goal in the 10th minute. Kelly O'Hara lofted a long pass down field that Kiwi defender Abby Erceg failed to control running back toward her own goal and Morgan ran onto it before calmly taking it round a stranded Jenny Bidon in the New Zealand goal. Usually so lethal, the U.S. forward dragged her shot wide of the unguarded net.
The chances kept coming the United States' way. This time it was Megan Rapinoe who broke clear and supplied a teasing low cross that somehow evaded Morgan in front of goal before Wambach, perhaps taken by surprise, side-footed wide at the back post. It was a clever move from the U.S. that deserved a better finish.
With New Zealand refusing to just sit deep, opportunities kept arising for the U.S. to get in-behind. Tobin Heath was the next to profit from a clever though ball by Wambach, but the gifted midfielder on this occasion supplied a poor cross with Morgan waiting in the middle.
The breakthrough would not take much longer to arrive. And there was little surprise that, as she was against North Korea, Morgan was once again the key architect.
Morgan twisted and turned Alex Smith down the left, before firing in a smart shot-come-cross to the far post that Wambach was on hand to prod past Bindon.
New Zealand's continued positivity meant that the U.S. did not have things all their own way, however. While their outlook failed to yield much threat to Hope Solo's goal in the opening period, Hassett shot from edge of box looped up and in the end was comfortable for Hope Solo.
Having almost profited from an Ercig error in the first half, Morgan came close to doing similar just minutes after the restart. The 23-year-old picked the ball off the defender to allowing her to close in on goal, but from the right of the area sliced her shot into the side netting at the near post.
Her finishing may not have been quite up to scratch, but Morgan was proving a constant menace to New Zealand's defenders. Looking to grab another opportunity for herself in the area, Morgan was bundled down from behind by New Zealand captain Rebecca Smith. The referee was unmoved, but the U.S. had every right to feel aggrieved at not being awarded a penalty.
Despite creating the goal for Wambach, it wasn't proving to be Morgan's day. Just past the hour mark Heath supplied the so-often available ball in-behind to find Morgan-who looked suspiciously offside-but the United States' top scorer in 2012 was prevented from adding to her tally as Bindon came out to block on the edge of the box.
Already having courted controversy for failing to award a penalty earlier, referee Jesica di Iorio was soon the center of attention once more. Morgan burst onto a through ball and knocked the ball past Bindon on the edge of the area, who slid into challenge, while Morgan tried to hurdle the goalkeeper she clattered into the face of Bindon and was left prone on the turf.
While Bindon's blow to the face muddied the decision, there can be little doubt that she prevented Morgan having a clear path to goal and should have seen red. To the dismay of the U.S. team, the referee merely awarded a drop ball.
Perhaps realizing her previous errors, or else merely continuing a poor day at the office, the referee denied New Zealand a good shout at a penalty in the final 10 minutes.
With New Zealand looking more menacing than they had done at any other stage in the match, the U.S. finally got the goal to secure victory. Substitute Sydney Leroux fittingly did what the women she replaced, Morgan, was unable to as he provided a cool finish after out-pacing and out-muscling her marker from a long ball down the left.
In the end, The U.S. progressed with comfort, but if they are to retain their Olympic gold then Sundhage knows that her team needs to be more clinical in front of goal.