Two first-half goals from Sophia Smith got the United States’ defence of the Women’s World Cup underway with a 3-0 win over Vietnam in Auckland.

But the tournament favourites were wasteful in front of goal, missing a penalty as they failed to turn their dominance – 28 shots to none – into a bigger win against outclassed but determined opposition.

Rising star Smith fired them ahead after 14 minutes, running on to Alex Morgan’s flick to fire a shot past goalkeeper Tran Thi Kim Tranh.

The US dominated the rest of the first half but failed to add to their lead – Julie Ertz, debutant Savannah DeMelo, Lindsay Horan, Smith and Morgan all having chances before Morgan wasted the best opportunity, Kim Tranh saving her weak spot kick given for a foul on Trinity Rodman.

The frustration grew further when Smith’s strike was ruled out for offside deep into stoppage time, only for VAR to rule Morgan was on the right side of the last defender and the lead was doubled.

Smith almost completed her hat-trick within seconds of the restart, sneaking onto a long ball through a hesitating defence only to put her effort wide.

Ertz later headed a DeMelo free-kick wide before Horan volleyed off target at the end of a sweeping move.

The introduction of Rose Lavelle and Megan Rapinoe, for her 200th international appearance, came just after the hour mark.

Horan finally added a third, firing in from near the penalty spot as Smith seized on hesitation in the Vietnam defence.

Lavelle came closest to a fourth late on as Kim Tranh’s fingertip turned her effort against the crossbar.