Veteran United States star Kelley O'Hara was close to tears as she contemplated the upcoming World Cup being Megan Rapinoe's last, vowing Tuesday to send the women's football icon out "on a high".

The 38-year-old Rapinoe, who is also well known for her off-field activism, will retire at the end of this season, bringing the curtain down on a 17-year career in which she has twice won the World Cup.

"It is hard to put into words," an emotional O'Hara said of her teammate from the 2015 and 2019 World Cup-winning sides, ahead of the tournament in Australia and New Zealand starting Thursday.

"The 'Pinoe' that the world sees is an incredible person, and that is her also up close and personal," O'Hara, who has won 157 caps, told a press conference in Auckland.

"She brings a sense of humour and lightness, but intensity and empathy, and just is one of a kind.

"There has never been one like her, there is probably never going to be one close to her.

"It is sad to think about this being her last, but she has done such incredible things for this team.

"I hope we all send her out on a high."

Rapinoe and the United States are chasing an unprecedented third World Cup crown in a row.

The forward is a former winner of the Ballon d’Or and the Best FIFA Women's Player awards.

She has been a high-profile campaigner on social issues including LGBTQ+ rights, racial inequality and gender and pay equality.

She was also a leading voice in the US women's team's successful fight for equal pay and conditions which resulted in a lawsuit and eventual new collective agreement being struck in 2021.

The United States on Saturday face outsiders Vietnam to open their title defence.