Venus Williams defeated Maria Irigoyen of Argentina 6-1, 6-4 on Sunday to advance the United States to the Fed Cup playoffs in April and a chance to return to the World Group, which comprises the top eight teams.
Williams' victory on outdoor clay gave the United States an unassailable 3-1 lead in the best-of-five World Group II tie.
Earlier Sunday Paula Ormaechea of Argentina defeated Coco Vandeweghe 6-4, 6-4. Vandeweghe filled in for Serena Williams, who did not play because of illness.
Venus and Serena gave the United States a 2-0 lead on Saturday, defeating Ormaechea and Irigoyen.
The Americans have won the Fed Cup title 17 times, more than any other nation, with the last victory in 2000.