Germany's Julia Goerges beat world number one Caroline Wozniacki for the second time in 11 days to reach the Madrid Open quarter-finals.
World number 27 Goerges followed up her with a 6-4 1-6 6-3 win over her Danish opponent.
World number 41 Bethanie Mattek-Sands stunned French Open champion and third seed Francesca Schiavone 7-6 (7-5) 6-3.
Fourth seed Victoria Azarenka beat Spain's Arantxa Parra Santonja 6-0 6-3.
But the result of the day in the women's event came in the opening match as 22-year-old Goerges, despite being hampered by a blister, again saw off Wozniacki on clay, leaving the Dane with only next week's Italian Open to prepare for the French Open at the end of the month.
Goerges made a huge leap in class as she beat Wozniacki to win in Stuttgart, only the second WTA title of her career, and back-to-back victories over the world number one will mark the German out as a dangerous opponent for anyone at Roland Garros.
She rallied from 3-0 down to take the first set and, after losing her way in the second, got the decisive break for 5-3 in the third before serving out the match.
"I did a pretty good match again and I'm proud of what I have achieved the last few weeks," reflected Goerges, who made 38 unforced errors compared to Wozniacki's 21 but also 36 winners to her opponent's 11.
"I beat her twice - and twice in two weeks - so I'm pretty proud of myself.
"I tried to get her out of her rhythm and not let her push me and be the first to push her. I like to play higher balls to her backhand especially and if she goes short I have a chance to go for the shots."
"The second set she started to be more aggressive and I started to get tired, so I said to myself keep yourself together for the third set and try to fight for every point."
Goerges will now meet Anastasia Pavlyuchenkova, who defeated fifth seed Sam Stosur 7-6 (7-4) 6-3.
American Mattek-Sands - sporting knee-high black socks to match the thick black eyeliner under her eyes - celebrated the biggest win of her career.
"I'm really happy with my play on clay right now," the 26-year-old said after defeating Schiavone. "I feel like a true dirtballer."
Her next opponent will be sixth seed and Australian Open runner-up Li Na of China, who beat Italy's Roberta Vinci 2-6 6-2 6-1.