The United States launched their bid for an unprecedented third consecutive world crown with a comfortable win against Women's World Cup debutants Vietnam in Auckland.

Sophia Smith struck twice before half-time for the four-time world champions in front of thousands of travelling USA fans in a crowd of 41,107.

Smith, one of 14 players in the squad appearing at their first World Cup, fired the number-one ranked team in the world ahead with a low-drilled finish from an angle.

Her second was awarded following a lengthy check by the Video Assistant Referee for an offside before captain Lindsey Horan, after an assist by Smith, swept home the third in the 77th minute.

There was a memorable moment for Vietnam keeper Tran Thi Kim Thanh when she was mobbed by her jubilant teammates after keeping out Alex Morgan's penalty, which was awarded for a foul on Trinity Rodman.

Vietnam rarely managed to get out of their own half but worked hard and produced a gritty defensive performance to keep the scoreline respectable.