A brace of tries by the hotstepper Rosko Speckman highlighted the Blitzboks' opening salvo in defence of their Dubai Sevens title as they dispatched Zimbabwe 31-0.
"There wasn't much said [before the final], said Knewstubb, who was named the HSBC Player of the Final".
Defending champions Australia ended England's hopes in the quarter-finals of the Dubai Women's Rugby Sevens. Scotland scored on fulltime to close the margin to 21-7. "The boys put in an unreal shift, I'm really happy".
The Red Roses made the ideal start to the competition with wins over Australia, USA and China to advance to the last eight where they will face reigning champions Australia on Friday.
The Scots ran in four tries as well as a penalty try against Kenya's two tries.
Final: New Zealand 21 (Tone Ng Shiu, Dylan Collier, Ngarohi McGarvey-Black tries; Andrew Knewstubb 2 con, Vilimoni Koroi con) US 5 (Stephen Tomasin try).
New Zealand were reduced to six at the start of the second half when Luke Masirewa was sin-binned for a high tackle, and a Martin Iosefo break drew the Kiwis to free Stephen Tomasin to score untouched for the Eagles.
John Tait's team will face Ireland in the Cup quarter-finals on Friday.
The first came from a chip by Justin Geduld and the second came with trademark speed at the end of the game. Ben O'Donnell scored their second after a lovely show and go, before a great step from Nick Malouf stepped back inside for the third.
The remaining match saw another unpredictable outcome as the Olympic champions and 2018 series runners-up Fiji were outdone by USA. However, Waisea Nacuqu had other ideas and his breakaway try levelled the scores, only for Vilimoni Botitu to step up and miss the straightforward conversion to mean extra-time would decide the victor.
Du Plessis expertly beat his defender on the outside for the opening try which gave his side the lead at half-time. The Canadian women have won 10 of 12 matches this season, losing only to the Black Ferns.
The Blitzboks claimed a 29-0 victory over Scotland in their fifth-place semi-final at the Dubai Sevens on Saturday.