Defending champion Angelique Kerber remains on track for back-to-back titles after easing into the Volvo Car Open quarter-finals with a straight-sets win over qualifier Kristina Kucova.

Top seed Kerber was a class above as she posted a 6-2 6-3 victory on the clay courts in Charleston on Thursday.

The world number two and Australian Open champion hit 35 winners to 33 unforced errors, while converting seven of 10 break-point opportunities in the routine victory against Kucova.

"It was a really tricky match," she said on court afterwards. "It was really windy and it was a completely different condition than before. I played two days ago in the night session, and it was windy, cold.

"I'm really happy to win the match because she's a tough opponent who had great matches in Charleston. I'm happy to be back here."

Next up for Kerber is high-flying Monica Puig, who saw off Irina-Camelia Begu 6-1 6-2.

American third seed Venus Williams saw her run at the WTA Premier event come to an end on Thursday, upstaged by Yulia Putintseva in a three-hour third-round marathon.

The seven-time grand-slam champion rallied to win the second set but Putintseva claimed five of the final six games in the decider to triumph 7-6 (7-5) 2-6 6-4.

Putintseva will face another seed in the last eight after Sara Errani outlasted 2010 winner Samantha Stosur 6-4 7-6 (7-5).

Sloane Stephens - the seventh seed - also won through to the quarters via a 6-4 6-3 victory over fellow seed Daria Gavrilova.

Russian teenager Daria Kasatkina accounted for Louisa Chirico 6-0 6-4, Mirjana Lucic-Baroni lost 7-5 6-2 to Laura Siegemund, and Elena Vesnina was too good for Lourdes Dominguez Lino 6-1 6-3.