Sportal.co.nz 24.Mar.2013Getty Images
The gallant Americans, powered by a brace to captain Matt Hawkins, pushed New Zealand hard into the final stages but could not hold on as they conceded two tries in the final minute of play.
Ben Lam, Trinity Spooner-Neera and Gillies Kaka all crossed the chalk, but it was Rocky Khan's final-minute effort that put New Zealand back in control after trailing 12-7 at half-time.
Having secured a play-off berth with a 31-7 thrashing of Kenya in the previous match, New Zealand brace for a tough quarter-final clash with Samoa.
As the overall series leaders, victory at Hong Kong would all but seal the 2013 title for New Zealand.
Their 96 points is well clear of nearest rivals South Africa (73), Samoa (71) and defending Hong Kong champions Fiji (66).
The gutsy win over a brave USA side will ensure they head into the knock-out stages battle-hardened and prepared for the challenge ahead.
There were concerns as the USA drew first blood through Hawkins in just the second minute.
But when Lam broke three tackles to get past the opposition defence, he showed outstanding pace to evade the chase of Carlin Isles to score under the posts.
However, the Aucklander's good work was undone when Trinity Spooner-Neera was sent to the sin bin shortly before half-time.
With a numerical advantage, the USA quickly worked their way into attacking territory where Hawkins dotted down again to put his side ahead 12-7.
In the second stanza, Spooner-Neera made amends for his earlier mistake with a try to level the scores.
Unable to slot the crucial conversion, Khan went about putting his side back on top with a stunning 50-metre burst to the goal line.
A late try to Hawke's Bay product Kaka put the icing on the cake.
New Zealand: 24
Tries: Lam, Spooner-Neera, Khan, Kaka
Cons: Khan 2
Tries: Hawkins 2