Canada's Eugenie Bouchard battled past Jelena Jankovic to reach the semi-finals of the Family Circle Cup.
Bouchard, who previously edged out Venus Williams, looked set for a straight-sets victory after taking the first set and breaking for 4-3 in the second but Jankovic rallied to force a third set.
While second seeded Jankovic's down-the-lines dazzled, the sixth seeded Canadian was consistently impressive on her return and duly closed out the Serb to win 6-3 4-6 6-3 in Charleston.
"I knew in the second set my game wasn't where I wanted it to be," Bouchard told the WTA Tour's official website.
"I think she did a good job of stepping in and controlling, and she changes directions a lot, so I was really on the run. And that's fine, but that's not where I want to be most of the time. I needed to get back to my game.
"I didn't really think about how I was up a break in the second, because it was in the past, and I had to move forward. I'm happy I didn't hold onto that. It was important to have short-term memory."
Awaiting Bouchard in the last four is Germany's Andrea Petkovic, who needed three sets to overcome ninth-seeded Czech hopeful Lucie Safarova 6-3 1-6 6-1.
Petkovic - seeded 14th here - has beaten Bouchard in their two previous meetings but the 20-year-old is now ranked 20th in the world, while the German is currently 40th.
In the other semi-final, Swiss qualifier Belinda Bencic will meet Jana Cepelova after they both upset seeds in their quarter-finals.
Bencic had to fight back from a set down to beat third seed Sara Errani but rattled off nine straight games to complete a 4-6 6-2 6-1 triumph.
"She played really well. I had problems in the beginning because it was our first time playing and I didn't know her game so well," said Bencic. "But then I adapted and stepped in for the spin balls.
"I just kept trying to focus on the next points and not the score, and I managed to play more and more aggressively and step in more. That was the key. I started serving better after the first set too."
Cepelova's win was far more straightforward as she overcame 12th seed Daniela Hantuchova 6-2 6-1.
"I'm really happy," she said. "It was a tough match. Daniela is a good player, and she's my good friend, so every time you play each other it's very tough. But I'm really happy I won here tonight."
The victory continued a remarkable tournament for the Slovakian who had already beaten world number one Serena Williams and 13th seed Elena Vesnina.