Cincinnati - Karolina Pliskova upset Angelique Kerber 6-3, 6-1 on Sunday to win the Cincinnati women's title and deny the German a chance to reach the WTA Tour's world number one ranking.
It was the second defeat in a final in little more than a week after Kerber lost to Puerto Rican Monica Puig in the gold-medal Rio Olympic match last weekend.
"I tried my best, but sometimes you have days where it's not working so good," Kerber said.
The Olympic silver medalist was broken early to start both sets as her 15th-seeded Czech opponent earned the upset.
The result leaves Serena Williams atop the ranking list with Kerber trailing in second.
Kerber, who had said all week she was keeping the number one possibility out of her mind, got off to slow starts in both sets.
She went down an immediate pair of breaks in the first set and lost her first service game in the second on the slippery slope to defeat.
The momentum mounted fast for Pliskova on a sunny day after a week of poor weather and a near-record 17 centimetres of rain. She lifted the trophy with 24 winners as she played in her 15th final
"I would like to congratulate Angie. The whole year she's doing a great job," Pliskova said. "I think you deserve to be number one, maybe next time."
The Czech, who now owns seven career titles and stands 3-4 against Kerber, claimed the opening set in 32 minutes and needed 62 minutes in total to complete her role as spoiler.
Pliskova is the first from her country to win the Cincinnati title. She finished with seven aces and five breaks of Kerber.
She won her second title of the season after claiming the honours last June on grass at the Aegon Open in Great Britain and is predicted to rise to 11th in the world with a week to go before the start of the US Open.
Kerber fell into a 5-1 deficit in the second set after a double-fault. Pliskova served out the biggest win of her career a game later on a second match point.