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The seventh-seeded 20-year-old blew away fellow American Lauren Davis 6-2, 6-2 in just 73 minutes on the clay courts in Charleston, South Carolina.
Keys routinely pounded serves in excess of 160 kilometres per hour (100 mph) and hit precision ground strokes for winners. She hammered four aces, had just one double fault and broke Davis four times.
"I think I was rushing a little bit at the beginning," Keys said. "After that I really kind of calmed down and just started playing the right balls, just being a little bit smarter on the court."
The 21-year-old Davis had stunned top-seeded Canadian star Genie Bouchard in straight sets on Wednesday.
Of the players remaining in the draw none are ranked in the top 10 in the world.
The Australian Open semi-finalist Keys' next opponent, on Saturday, will be Czech qualifier Lucie Hradecka, who upset fourth-seeded former French Open runner-up Sara Errani 6-2, 6-4.
Since reaching the semis in Melbourne, Keys was eliminated early in her only two tournaments, at Indian Wells and Miami.
The start of the clay season has been good for her game.
"It's been a pretty good week," she said. "I haven't had many complaints about how I've been playing. I'm not trying to get too high on myself, I'm just trying to keep playing how I've been playing."
The last match of the night saw fifth-seeded Angelique Kerber topple 13th-seeded Irina-Camelia Begu 7-6 (7/3), 7-6 (7/4).
Kerber's opponent in the semi-finals will be third-seeded Andrea Petkovic, who rallied from a first-set loss to down Danka Kovinic 2-6, 6-3, 6-1.
Petkovic defeated the Slovak Jana Cepelova in last year's final.
Results on Friday in the WTA claycourt tournament at Charleston (x-denotes seeding):Quarter-finals
Madison Keys (USA x7) bt Lauren Davis (USA) 6-2, 6-2
Lucie Hradecka (CZE) bt Sara Errani (ITA x4) 6-2, 6-4
Andrea Petkovic (GER x3) bt Danka Kovinic (MNE) 2-6, 6-3, 6-1
Angelique Kerber (GER) bt Irina Begu (ROM) 7-6 (7/3), 7-6 (7/4)
