Wales hit the lead early in the second spell after USA were reduced to six players. Rhys Jones touched down and banged over the conversion to give Wales a 10-7 lead.
Wales held their lead until Zachary Pangelinan pounced to score in the ninth minute to rock Welsh hopes of reaching the semi-finals.
Wales then produced a flat performance in the semi-final of the Plate competition against South Africa to go down 29-0.
After five legs Wales are currently in 10th place on the points table with 24 points with New Zealand leading the way with 96 points, followed by the Adelaide winners Samoa who have accumulated 94.
Wales now head to Hong Kong for the next leg where they have drawn England, Japan and Hong Kong.