Cincinnati - Karolina Pliskova upset fourth seed Garbine Muguruza of Spain 6-1, 6-3 on Saturday to move into the final of the Cincinnati Masters.
The 15th seed from the Czech Republic, who fired her 400th ace of the season in the final game, got through just in time as the skies erupted with rain only minutes after the victory.
Pliskova will await a Sunday showdown against the winner from the second semi-final between Olympic silver medallist Angelique Kerber and the Romanian Simona Halep.
Should Kerber win the title on Sunday she would seize the world number one ranking from Serena Williams, who has held it for the last 183 weeks.
At 17 in the world, Pliskova is the lowest-ranked finalist at Cincinnati since 2008, when number 21 Natalie Petrova won the title.
Muguruza, the French Open champion, was overwhelmed with just seven winners and 23 unforced errors.
"'m happy with how I played today," said Pliskova. "The whole week it has been hot and humid, Today it was windy and not that hot.
"I'm glad to win a good match. I tried to play aggressive, she has a similar game to me.
"I knew I had to make the pressure and concentrate on my serve."