Prior to her arrival in Washington, Rybarikova had registered just one win in her past three tournaments. However, after a slow start in her first match, the Slovak has looked increasingly at home, and produced her best tennis of the week to see off Kerber, 76(0) 36 63, for her second career Top 10 victory.
"I don't really know why I play so well here," Rybarikova said. "I like the court: it is not too fast, like most American courts, and I feel very comfortable here."
Next up for Rybarikova is Ekaterina Makarova, who enjoyed a surprisingly easy passage through to the semifinals when her opponent, Monica Niculescu, retired with a left wrist injury after the opening game.
Following comfortable wins against Eugenie Bouchard and Caroline Garcia in the opening two rounds, Makarova has now spent just two hours and 15 minutes on court at the William H.G. Fitzgerald Tennis Center this week.
Rybarikova's progress, on the other hand, has not been quite so smooth. "Last year was amazing, I won every match in straight sets, but this year I have always gone to three sets and now I'm pretty tired!" Rybarikova added. "I'm just trying to relax and just play my game.
"It's going to be very tough because Ekaterina hardly even played today! Also before this, she won both her matches very quickly and very easily. She's playing well and is the favorite, but I'm confident and will just try and enjoy it."
In the bottom half of the draw, Andrea Petkovic continued her recent upturn in fortune with a 46 61 63 victory over Paula Ormaechea. Petkovic defeated Ormaechea easily en route to the ITF Circuit title in Marseille this June, but struggled to find her range in the opening set.
"When I played her in Marseille, we were playing on clay and she played completely differently," Petkovic said of her decision to call for her coach at the end of the opening set. "Today she came out blazing hot, hitting winners all over the court and serving great, so it was tough!
"I was a little off with my footwork and so wasn't able to take control of the points, which made a difference in the first set. My coach said I needed to try and step in and be more aggressive and that's what I did and slowly managed to take over."