Stuttgart - Roland Garros champion Jelena Ostapenko continued to build for her Paris title defence with a 6-3, 6-0 win over Zarina Diyas on Thursday at the Stuttgart Grand Prix.
The Latvian was joined in the quarter-finals at the Porsche arena by fifth seed Karolina Pliskova as the Czech beat Russian qualifier Veronika Kudermetova 7-6 (7/4), 6-3.
Ostapenko, ranked fifth on the WTA rankings, broke her 52nd Kazakh opponent five times as she won in just 63 minutes.
"The early matches are tough but I think I played really well today and I'm just happy with the win," Ostapenko said.
The 20-year-old added that said she is still adjusting to life as a Grand Slam champion.
"I expected I could win a Grand Slam but not at this early age.
"I just understood a couple of months later. Of course there is more attention from everyone. I got more popular especially in my country and almost every match I played I was the favourite.
"At the beginning of this year, it was a little bit tough, a lot of pressure but I think I got used to it; especially after the Miami final (which she lost to Sloane Stephens) I'm feeling much better."
Pliskova hammered 15 aces as she beat Kudermetova in a match that lasted just past the 90-minute mark.
Tournament debutant and third seed Elina Svitolina advanced as Czech qualifier Marketa Vondrousova retired trailing 2-6, 6-1, 3-2 in their third-round contest.
"I always need a few weeks to switch form hardcourt to the clay," the winner, who was a French Open quarter-finalist in 2015, said. "I am just try to east into it, it's always difficult for me."
American CoCo Vandeweghe extended her unexpected run of clay form, beating defending champion Laura Siegemund of Germany 6-4, 4-6, 6-3.
The Californian, ranked 16th after breaking into the top 10 last season, said her secret appears to be playing relaxed.
"I probably have more fun on clay since I don't care too much about it," she said prior to only the second clay quarter-final of her career where she will face top seed Simona Halep.
Carolina Garcia, a first-round winner over three-time champion Maria Sharapova, knocked out 15-year-old Ukrainian prodigy Marta Kostyuk 6-1, 3-6, 7-5
Results from the WTA Stuttgart Grand Prix on Thursday (x denotes seeding):
Jelena Ostapenko (LAT x4) bt Zarina Diyas (KAZ) 6-3, 6-0
Karolina Pliskova (CZE x5) bt Veronika Kudermetova (RUS) 7-6 (7/4), 6-3