Madison Keys has rallied late to down top-seeded Sloane Stephens for the first time in four career meetings to reach the semi-finals of the WTA event in Charleston.

Keys pulled away from her fellow American to win 7-6 (6) 4-6 6-2.

“It’s always tough to play a friend and it kind of shows at times like that,” Keys said.

“It was up and it was down and it was just kind of crazy.”

Stephens had not lost a set in three previous matches against her good friend, including in the 2017 US Open and last year’s French Open before dropping the opening set tiebreaker.

Keys, seeded eighth and ranked 18th in the world, dug in once again down the stretch after Stephens won the second set.

She will take on Monica Puig of Puerto Rico for a spot in Sunday’s final.

Puig crushed 29 winners to topple American Danielle Collins in their quarter-final to end the day.

The 25-year-old Olympic gold medalist from San Juan fired down six aces to book her first semi-final trip since the Connecticut Open in August.

No.5 seed Caroline Wozniacki of Denmark, the 2011 Charleston champion, also made the final four after defeating 15th-seeded Maria Sakkari of Greece 6-2 6-2.

Wozniacki smacked 15 winners to just eight unforced errors and looked particularly sharp returning serve, where she moved briskly and attacked Sakkari’s second serves.

She will face No.16 seed Petra Martic of Croatia, who ousted ninth-seeded Belinda Bencic of Switzerland 6-3 6-4.