Madison Keys overcame Simona Halep for the first time in five years in a dramatic match at the Western and Southern Open in Cincinnati.
The American number-16 seed beat the fourth seed 6-1 3-6 7-5 in a match that lasted almost two hours and delighted the capacity crowd.
In doing so, Keys reached the last eight in Cincinnati for the second straight year.
Russia's Svetlana Kuznetsova will join Keys in the quarter-finals after beating American Sloane Stephens 6-1 6-2.
World number one Naomi Osaka beat Hsieh Su-wei of Taiwan in another three-set thriller, 7-6 5-7 6-2.
In other games, Karolina Pliskova of the Czech Republic defeated Sweden's Rebecca Peterson 7-5 6-4, Venus Williams overcame Donna Vekic 2-6 6-3 6-3 and Australian Ashleigh Barty won against Estonian Anett Kontaveit 4-6 7-5 7-5.
There were also wins for Maria Sakkari of Greece, who beat Aryna Sabalenka 6-7 6-4 6-4, and America's Sonya Kenin, who beat Ukranian Elina Svitolina 6-3 7-6.