South is ranked four in Britain and 154 in the world
Melanie South followed in the footsteps of Jo Durie and Anne Hobbs by becoming only the third Briton to reach the quarter-finals of the DFS Classic.
The Briton followed up her second-round win over fourth seed Sybille Bammer in Birmingham with a 2-6 6-4 6-4 victory over world number 78 Aiko Nakamura.
The 22-year-old from Surrey overcame a five-hour rain delay and a sluggish start to win in one hour 43 minutes.
South will meet 18-year-old Yanina Wickmayer of Belgium in the last eight.
Wickmayer created the latest upset of a tournament of shocks by putting out eighth seed Michaella Krajicek of the Netherlands, 6-3 6-4.
Title favourite Nicola Vaidisova booked her spot in the quarters with a laboured 6-7 (5-7) 6-2 6-4 win over Russian Ekaterina Makarova.
The world number 19, who is now coached by Tim Henman's former mentor David Felgate, faces American Bethanie Mattek next.
South said self-belief and determination were the factors that enabled her win through.
"I started a little bit nervous but I still had belief that I could turn it around," she said.
"She played really well in the first set. It was pretty tough and I was just hanging in there but I managed to play really good tennis in the end.
"I knew I had to stay with her. She's good on grass and has had some good results. Even when she hit some good shots, I didn't let it faze me.
"Obviously it was a big match for me, getting to the quarter-finals. It did mean a lot to me and that got me through in the end."