Stephens had not lost a set in three previous matches against her good friend, including in the 2017 U.S. Open finals 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.
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