CHARLESTON, S.C. (AP) — Germany's Julia Goerges continued her strong season by defeating defending champion Daria Kasatkina to reach the semifinals of the Volvo Car Open on Friday.

Goerges, the fifth seed who's ranked No. 13 in the world, overpowered the third-seeded Kasatkina of Russia 6-4, 6-3 with nine aces — she won 84 percent of her points on her first serve — to advance to the final four at the WTA's opening clay-court tournament.

Goerges, who won earlier this year at the WTA's New Zealand event, will face No. 8 seed Anastasija Sevastova of Latvia, who ended the surprise run of Kristyna Pliskova with a 6-4, 6-0 victory.

American Madison Keys, the U.S. Open runner-up last year, also advanced, prevailing in an all-U.S. match with Bernarda Pera 6-2, 6-7 (5), 7-5.

Keys will take on No. 12 Kiki Bertens of the Netherlands, who defeated No. 14 Alize Cornet of France 6-2, 7-5.

Goerges said getting her serve going early was crucial to beating Kasatkina.

"I served pretty big today, which was also (a) good key, of course," Goerges said. "If you have good service games, you can put more pressure on the return games, and I think that was a good combination."

Kasatkina, 20, had won her past eight matches here and got emotional when that run ended in her first defense of a WTA crown.

"I was not able to hold my tears after the match," she said. "It's tough, but I'm happy that I have this experience now, and I went through this and I'm pretty sure next time is going to be much easier."

Little came easy for Keys the final two sets against Pera, the 23-year-old ranked 101 in the world yet won three matches to make the quarterfinals.

Keys had match point in the second set, then was down a break to Pera in the third. Keys said her coach, Lindsay Davenport told her, "'I don't care what happens, just show me that you want it.' And that was really all that I had to focus on, and I think that brought my level up."

Sevastova, ranked 16th, outlasted Pliskova in the opening set before winning the second one going away.

Pliskova, the twin sister of top-10 player Karolina Pliskova, had upset two-time Wimbledon winner Petra Kvitova and No. 16 seed Elena Vesnina to make the round of eight.

Bertens has not lost a set all week on the green clay at the Family Circle Tennis Center. She broke Cornet's serve seven times in the victory. Bertens, who reached the French Open semifinals in 2016, felt the different surface has helped her game.

"The clay is helping me a little bit, like with my forehand and with my heavy strokes, but I guess it's more mentally," she said. "My level is maybe 20 percent better, but in my head I feel like I'm 80 percent better."