Keys pulled away from her American countrywoman to win 7-6 (6), 4-6, 6-2. Keys, seeded eighth and ranked 18th in the world, will take on Monica Puig of Puerto Rico for a spot in Sunday's finals. The unseeded Puig defeated her third consecutive seeded opponent, topping No. 11 Danielle Collins of the United States 6-3, 6-2.
Stephens had not lost a set in three previous matches against her good friend, including in the 2017 US Open final and last year's French Open semifinals, before dropping the opening-set tiebreaker. Keys dug in once again down the stretch after Stephens won the second set.
No. 5 seed Caroline Wozniacki of Denmark, the 2011 Volvo champion, also made the final four after defeating 15th-seeded Maria Sakkari of Greece 6-2, 6-2. Wozniacki will face No. 16 seed Petra Martic of Croatia, who ousted ninth-seeded Belinda Bencic of Switzerland 6-3, 6-4.