Gauff was the only American woman to make it into this year's Cincinnati last eightAmerican Coco Gauff shocked world number one Iga Swiatek in a thriller to reach the Cincinnati Open final.
Gauff had never managed to win a set in seven previous matches against the Pole, but showed huge determination to win 7-6 (7-2) 3-6 6-4.
The 19-year-old dominated the first set tie-break but Swiatek broke early in the second set and levelled the match.
But Gauff grabbed her first break-point chance in the decider and held on for the biggest win of her career.
She will face either second seed Aryna Sabalenka or French Open finalist Karolina Muchova in Sunday's final.
Neither Gauff nor Swiatek was able to take control of an opening set that included four consecutive breaks of serve but the American seventh seed hustled her way into a 4-0 lead in the breaker and held on to take it.
It looked like normal service was resumed for Swiatek in the second set as she struck early for a 2-1 lead thanks to some heavy groundstrokes and maintained her dominance.
Serving first in the decider, Swiatek was barely troubled in her first three service games.
But at 3-3, she started to show some signs of vulnerability and Gauff converted the second of two break points in the game to edge ahead.
Swiatek missed a break-back opportunity in the next game but then held to put the pressure back on Gauff to serve out for the match.
There were some nervy moments for the teenager as she struggled with her serve and Swiatek saved three match points but she could do nothing with the fourth as Gauff celebrated a famous win.
