Keothavong is the fourth seed in Memphis
Anne Keothavong reached the quarter-finals of the RMK Championships after a fine 7-6 (7-2) 6-4 win over Portugal's Michelle Larcher De Brito in Memphis.
The British number one edged the first set on a tie-break but needed all her resolve to fight back in the second.
The Portuguese 16-year-old led 4-0 but Keothavong stormed back with six successive games to seal the victory.
The 25-year-old Londoner will face fifth seed Marina Erakovic from New Zealand in the last eight.
The tournament's top seed Denmark's Caroline Wozniacki beat France's Severine Bremond 6-1 7-6, while the third seed Lucie Safarova beat American Vania King 7-6 (7/1) 6-3.
In Wednesday's other games, Germany's Sabine Lisicki saw off Stephanie Foretz from France 6-2 6-4 and French eighth seed Pauline Parmentier overcame South African Chanelle Scheepers 6-4 6-1.
In the men's event, Lleyton Hewitt saw off third seed James Blake 3-6 6-1 6-4, while top seed Andy Roddick beat Belgian Steve Darcis 7-6 (7-1) 6-2.