In a battle between qualifiers, Riske rolled past Alla Kudryavtseva in just 54 minutes, 61 62, losing just 14 points in eight service games in the match and breaking Kudryavtseva's serve four times.
This is the third time Riske has reached the quarterfinals or better at the WTA level and it can't be a coincidence anymore - all three times have come right here, having also done it in 2010 (losing to Maria Sharapova in the semifinals) and in 2011 (falling to Daniela Hantuchova in the quarterfinals).
Riske wasn't the only American to reach the quarterfinals of the grass court event, with Madison Keys pulling off a 63 62 surprise over No.6 seed Mona Barthel. Like Riske, Keys was impressive on her serve - she held all nine of her service games - and she snuck out three breaks against the German.
"No match on grass is easy because things can change so quickly, but I was really happy I was able to get on and get off," Keys said afterwards. "I think I was really solid off the baseline today, which really helped me play some good return games and break her. And I also served really well today."
Keys is also through to her third WTA quarterfinal, though she has done it at all different events - her first two came at Sydney and Charleston earlier this year. And all on different surfaces, too.
"I was just hoping to get a couple matches on grass courts this week and get used to them a little bit more, so I'm really happy to get to the quarterfinals here," Keys said. "The courts are very nice. It bounces pretty low and occasionally you get an awkward bounce but overall they're really good courts."
It's taking a little bit of adjustment for the American teenager to go from clay to grass, too.
"The toughest thing is probably the movement," Keys said. "I feel like I keep trying to slide into the balls here! You play on clay for so long, so you have to remind yourself you're not on clay anymore.
"But I love the grass because it's a lot faster. I think it suits my game much better."
Keys wasn't the only player to spring an upset, with Daniela Hantuchova rallying from 2-0 down in both the second and third sets and surviving 14 aces to take out No.13 seed Kristina Mladenovic, 36 76(6) 64, and No.16 seed Magdalena Rybarikova saving five set points late in the first set then eventually rolling to a straight set victory against the tournament's No.1 seed, Kirsten Flipkens, 76(5) 62.
Elsewhere, No.3 seed Sorana Cirstea won a bizarrely quick three-setter against Bojana Jovanovski in just an hour and 12 minutes, 61 16 61; No.5 seed Sabine Lisicki rallied from a break down in the third to beat Mirjana Lucic-Baroni, 75 26 64; No.15 seed Francesca Schiavone dug out of a 5-2 first set hole to beat Marina Erakovic, 76(3) 63; and Donna Vekic beat American qualifier Maria Sanchez, 46 62 62.