Top seed Ashleigh Barty moved into the quarter-finals with a three-set win over Shelby Rogers while Petra Kvitova and Garbine Muguruza exited in the Round of 16 of the Volvo Car Open in Charleston.
World number one Barty was pushed all the way by 52nd ranked Rogers, with both players trading breaks in the final set, before the Australian prevailed 7-6 (7-3) 4-6 6-4.
The victory extends Barty's winning streak to eight matches having won last week's Miami Open.
Barty won 82 per cent of first serve points and sent down 10 aces to get the edge on the American, who saved eight of 13 break points throughout the match.
"The conditions are difficult. It's pretty quick," Barty said in her on-court interview post-game.
"Shelby was able to control the centre of court for a lot of the match. It was about trying to get control when I could. She's an exceptional player, it's always a fight no matter what surface or where we are in the world. It's always one that I enjoy."
Barty will play Spaniard 71st ranked Paula Badosa in the quarter-finals after she beat Caty McNally 6-3 6-3.
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Third seed Kvitova was the major casualty on Thursday, losing 6-4 6-1 to Montenegrin Danka Kovinic.
The first set was tightly fought before Kovinic pulled away as Kvitova's errors mounted.
"We both have pretty big serves and she's very good hitting from the baseline," Kovinic said post-game. "Today it was the key that I tried, and did it really well, to take her pace of the ball and use her energy."
Sixth seed Garbine Muguruza also bowed out injured, despite leading 6-0 2-2 against 11th seed Yulia Putintseva.
Muguruza laboured for a few games after a medical time-out before retiring with a left leg injury. Putintseva will next face Kovinic.
Seeds Ons Jabeur, Veronika Kudermetova and Cori Gauff also progressed to the last eight, while Sloane Stephens won in straight sets against Ajla Tomljanovic.
Meanwhile at the Copa Colsanitas second round in Bogota, seventh seed Tereza Martincova was knocked out, going down 6-3 6-3 to Colombian teenager Maria Osorio Serrano.
On the outdoor clay courts, fifth Tamara Zidansek moved into the quarter-finals with a 6-2 6-3 win over Giulia Gatto-Monticone, while Harmony Tan beat Daniela Seguel.