Alyssa Thompson was just a starry-eyed 14-year-old when she watched the 2019 Women's World Cup final on television at her aunt's house -- now the United States forward is playing at the tournament.

The Los Angeles native came on for her World Cup debut as a late substitute in the holders' 3-0 win over Vietnam in Auckland on Saturday.

She was just 14 when now-teammate Megan Rapinoe and the Americans won 2-0 against the Netherlands -- the United States' opponents on Thursday -- in 2019 to retain the trophy.

"I was at my aunt's house and I was watching it there because we didn't have TV at home," Thompson, now 18, told reporters in Auckland.

"I was surrounded by my whole family, we were watching the game and I just remember how intense the game was, how back and forth it was, and I was really nervous for the team."

Goals by Rapinoe and Rose Lavelle gave the USA their second straight title and their fourth overall that day.

They are favourites to retain the trophy this year in Australia and New Zealand despite Thompson being one of 14 players on coach Vlatko Andonovski's roster appearing at the finals for the first time.

"In that moment I wasn't really thinking that the next one I was going to be at because I felt so young, so it felt so far away," said Thompson, who plays her club football for Angel City.

"But being here now is crazy to me," added Thompson, who made her full international debut against England at Wembley in October last year.

She came on for her World Cup debut against Vietnam with the USA leading 2-0 thanks to two Sophia Smith goals.

Moments after Thompson's introduction, Smith set up skipper Lindsey Horan to secure a comfortable victory in the Group E opener.

"It felt amazing," Thompson said of her first World Cup experience.

"When I was warming up I was pretty nervous to play my first World Cup game but once I was on the sideline I was overcome with excitement and happiness.

"I wasn't feeling any other emotion so I couldn't help but smile. It was really cool."