Veteran Mirjana Lucic-Baroni booked her spot in the Volvo Car Open semi-finals as Caroline Wozniacki missed out on a top-10 berth.
Australian Open semi-finalist Lucic-Baroni rallied to beat hometown favourite Shelby Rogers 6-7 (7-9) 6-1 6-1 on Friday.
The oldest player left in the draw, having first played in Charleston in 2001 when Rogers was a ball girl during the tournament, 35-year-old Lucic-Baroni had two set points in the opening set but she was twice denied by Rogers, who triumphed in a tie-break.
But Lucic-Baroni – who broke five times in a row and reeled off 10 consecutive games – hit back relentlessly in the second and third sets to reach the final four.
Awaiting Lucic-Baroni is unseeded Jelena Ostapenko, who upstaged fifth seed and 2011 champion Wozniacki on the green clay courts.
A win would have seen former world number one Wozniacki return to the WTA's top 10, however she went down 6-2 6-4.
Ostapenko stormed out of the blocks, breaking serve three times to win six successive games in the opening set.
The 19-year-old Latvian started the second set with a break and while she handed is straight back, she proved too good for Wozniacki.
Laura Siegemund and Daria Kasatkina will meet in the other semi-final following their respective victories.
Siegemund was too good for eighth seed Anastasija Sevastova 6-2 6-4, while Kasatkina stunned 10th seed Irina-Camelia Begu 6-4 6-1.