Sloane Stephens reached the final of the Volvo Car Open after top seed and defending champion Angelique Kerber retired from their semi-final due to illness.
Australian Open champion Kerber decided she could not continue when trailing seventh seed 6-1 3-0 in Charleston on Saturday.
Top seed Kerber will lose her status as world number two to Agnieszka Radwanska as a result of missing out on a place in the final, while Stephens is just one win away from her third title of the year.
Stephens won the opening four games of the match to stun Kerber and the world number 23 wasted no time in wrapping up the opening set as the German looked out of sorts.
The American produced some eye-catching groundstrokes and applied pressure on the favourite's serve, but it was apparent that Kerber was struggling as she racked up 16 unforced errors and was unable to continue.
Stephens will face qualifier Elena Vesnina in Sunday's final of the WTA Premier event.
Vesnina sealed her place in the decider with a surprise 6-4 4-6 6-2 victory over Sara Errani, becoming the first qualifier to reach the final in the tournament's 44-year history.
The Russian outsider won 65 per cent of points on her first serve and broke six times to set up a showdown with Stephens at the expense of the fifth seed.