Unseeded Elena Vesnina will take on world number one Caroline Wozniacki in the final of the Family Circle Cup in Charleston.
The Russian, ranked 56th in the world, beat China's Shuai Peng 7-6 (7-4) 6-3 in Saturday's semi-final.
Earlier, top-seed Wozniacki beat world number eight Jelena Jankovic 6-4 6-4.
The Dane broke the Serbian five times but also suffered three breaks of serve herself in the hard-fought match.
The 20-year-old eventually battled on to win in one hour 47 minutes.
Their semi-final will be remembered for a succession of grinding rallies and numerous breaks of serve.
Each of the first three games went to deuce in the energetic opening stages. Jankovic looked to be heading for the first set as she gathered a 4-3 lead, but her opponent won three straight games to establish her lead.
In the second set, the Dane capitalised on Jankovic's errors.
"I had to be aggressive and hit it deep because [Jankovic] is such a good fighter," she said.
It was this aggression and an ability to return the seemingly un-returnable which eventually won out.
"She always makes you play an extra ball," third-seeded Jankovic said.
"Some other players, you think it will be a winner. With her, it somehow comes back."
Vasnina should not cause too many problems for Wozniacki who is chasing her third tournament victory of the year after her triumphs at Indian Wells and Dubai.
Yet this tournament has already seen the underdogs shock the South Carolina crowd in earlier rounds - Vesnina dumping defending champion Sam Stosur out at the quarter-final stage, while Peng caused an upset when she beat second seed Nadia Petrova.
Wozniacki, meanwhile, has made hard work in getting to the final. In the third round, she was kept on the court for two-and-a-half hours against the Czech world number 48 Barbora Zahlavova Strycova, eventually winning 7-6 (8-6) 7-6 (11-9).
She will take heart from her semi-final display, however - former world number one Jankovic had not dropped a set in the tournament until Saturday's semi-final.
"I'm just happy to be in the finals again," the Dane said. "It's been a great week for me."
In Marbella, top seed Victoria Azarenka moved a step closer to her second successive WTA tour title with a comfortable semi-final win over Sara Errani.
The 21-year-old Belarussian beat her Italian opponent, seeded eight, 6-2 6-1 at the Andalucia Tennis Experience.
Azarenka will face Romanian qualifier Irina-Camelia Begu in Sunday's final.
Begu, ranked 137 in the world, came from behind to defeat second seed Svetlana Kuznetsova of Russia 3-6 7-6 (7-3) 6-4.
The 20-year-old, who has 14 titles from the ITF circuit but had never previously won a main-tour event before arriving in southern Spain, dropped her serve seven times during the match but held her nerve to outlast her more experienced opponent.