Coco Vandeweghe of the U.S. in action against Kristina Mladenovic of France during their match of the Fed Cup World Group semi final tie between France and the United States in Aix-en-Provence, southern France, 21 April 2018.
They will defend their title on November 10-11 against the Czech Republic, who reached the final by defeating Germany 4-1 in Stuttgart.
Stephens, ranked ninth, needed to save five set points down 5-4 in the first set as she held on to beat Pauline Parmentier 7-6 (3), 7-5 in a match that lasted almost two and a half hours.
The reverse singles are on Sunday, followed by a doubles featuring Mladenovic and Amandine Hesse against Madison Keys and Bethanie Mattek-Sands. "We are very fortunate to have such a strong group and now we are looking forward to what is going to be a great final", USA captain Kathy Rinaldi said.
Earlier in the day, U.S. Open champion Sloane Stephens thrashed Mladenovic 6-2 6-0 to collect her second singles victory this weekend.
Stephens saved five set points serving at 5-4 down in the first set, and rallied from 4-3 and a break down in the second set against Parmentier.
Playing on clay indoors at the Arena Pays d'Aix, the players each had more than 50 unforced errors in the match. "You never anticipate a scoreline like the way it turned out but it was really solid", said Stephens, who earned two points for the USA team over the weekend in the southern French city.
The Americans hold an 11-2 winning record against the French, who won their most recent meeting in 2014.
The U.S. beat Belarus 3-2 in the 2017 final.