Defending champion Daria Kasatkina lost at the quarter-final stage of the Charleston Open on Friday, going down in straight sets to Julia Goerges.

The German reached the last four of the tournament for the first time in her career with a 6-4 6-3 victory over Kasatkina, seven years on from her last appearance at this stage.

Goerges, who has already knocked out Indian Wells champion Naomi Osaka, served nine aces, 33 winners and won 84 per cent of the points behind her first serve in a ruthless display of power.

"The key was to be patient and wait for opportunities to be aggressive. I'm an aggressive player but sometimes, I need to wait two or three more balls," she said.

"I improved a lot since I played my first match here, and it sometimes happens in a tournament that you don't feel well, but you manage to find a way to get through it, and then you never know where it's going to end."

Eighth seed Anastasija Sevastova will meet Goerges in the semi-finals after dispatching Kristyna Pliskova 6-4 6-0.

Madison Keys survived a gruelling 6-2 6-7 (5-7) 7-5 win over Bernarda Pera in an all-American encounter, surging to victory in the final set after squandering two match points. She will face Kiki Bertens, who beat Alice Cornet 6-2 7-5.

In Monterrey, Garbine Muguruza continued her dominant form with a 6-3 6-0 quarter-final win over Ajla Tomljanovic of Australia.

The world number three, who is yet to drop a set in Mexico, will meet Ana Bogdan in the last four after her 6-2 6-4 win over Miami Open semi-finalist Danielle Collins.

Sachia Vickery, who stunned second seed Magdalena Rybarikova 6-0 6-3, has Timea Babos next up, the Hungarian having eased past Monica Puig 6-4 6-2.