Two tries from Desiree Miller and Caitlyn Halse had put Australia in the ascendency, after leading 14-5 at half time, but quickfire scores USA's Hope Rogers Erica Jarrell-Searcy put the USA ahead with nine minutes to go.
Loraini Senivutu, Manuqalo Komaitai and Karalaini Naisewa all crossed for Fiji, but a red card for Bitila Tawake denied Cunningham's side a chance to mount an impressive comeback.
The wing was one of five try-scorers for the world number two side, with McKinley Hunt crossing twice, and Alysha Corrigan, Taylor Perry and Brittany Kassil also scoring.
Quinta do Lago's multi-sports hub and one of Europe's premier sporting facilities, The Campus, is launching its first ever HYROX Fitness Retreat this Autumn.