One of six members of the USA starting line-up making her World Cup debut, lively forward Smith finished a fine move to open the scoring in the 14th minute at Eden Park in Auckland.

Veteran Alex Morgan was then denied from the penalty spot before Smith struck again deep in stoppage time at the end of the first half in front of a crowd of just over 41,000.

She then set up the third goal for captain Lindsey Horan in the 77th minute, after veteran icon Megan Rapinoe had come off the bench to win her 200th cap.

"I think it was a good starting point for our team in this tournament. I also know that we have a lot more that we can give, a lot more to do, little things to work on," said the 22-year-old Smith.

Vlatko Andonovski's title favourites missed a hatful of chances as debutants Vietnam avoided the humiliation that befell Thailand at the 2019 World Cup, when they were thrashed 13-0 by the United States.

There is clearly room for improvement for an American side who will also play Portugal and 2019 runners-up the Netherlands in Group E, as they aim to win an unprecedented third consecutive title.

However, this was a new experience for many of their side, and for Andonovski, taking charge of a World Cup game for the first time having replaced Jill Ellis as coach not long after their triumph at France 2019.

"Unfortunately we didn't capitalise on all the great opportunities that we created and that is something we are certainly going to focus on in the lead-up to the next game, but the first one is in the books and the focus immediately shifts towards game two," said Andonovski.

That will be against the Dutch on Thursday in a re-run of the 2019 final.

With Rapinoe beginning her final World Cup before retirement on the bench, Smith and 21-year-old Trinity Rodman flanked Morgan in attack on their debuts at the finals.

Right-back Emily Fox, centre-back Naomi Girma, and midfielders Andi Sullivan and Savannah DeMelo also made their first World Cup appearances.

"I have to say I was happy with the way the team played. I think that if you look at the team it is the first time this XI have been on the field together," the coach added.

"To see some of the connections and combinations was very positive."

- Morgan squanders spot kick -                  

Rodman, the daughter of former NBA star Dennis Rodman, needed lengthy treatment after suffering a knock inside the first minute before going on to have an impact on proceedings.

It was Smith who got the breakthrough though, controlling and finishing off a fine move expertly after Morgan flicked on a pass by Horan.

It was one-way traffic but it remained 1-0 until Rodman was clipped by Hoang Thi Loan in the area just before the break and the Moroccan referee gave a penalty after coming over to review the images. 

Yet Morgan, playing in her 19th World Cup match, saw her poor kick saved by goalkeeper Tran Thi Kim Thanh diving to the right.

Instead Smith made it 2-0 towards the end of eight added minutes in the first half, firing through the legs of the goalkeeper and in after a Girma cross had been punched away.

The goal was initially ruled out before being given after another lengthy check confirmed there was no offside.

Rapinoe, her hair dyed blue, entered the fray to cheers just after the hour mark alongside Rose Lavelle, the two goal-scorers from the 2019 final. 

Teenager Alyssa Thompson also came on for her World Cup bow just before Smith pounced on a Julie Ertz ball forward and supplied a cutback for Lyon midfielder Horan to score.

Lavelle hit the underside of the bar late on as Vietnam escaped with a creditable scoreline on their World Cup bow.