Bertens, rebounding after a quick first-set loss, outlasted Romania's top seed in an exhausting second set tiebreaker, then won five of the final six games in the decider to lift her sixth career title, after five previous triumphs on clay.
"I cannot find words for this moment. I'm so happy," a tearful Bertens said on court.
The victory was the 26-year-old Bertens' first ever over a world number one and her sixth consecutive triumph over a top-10 opponent. It will see her move up to number 13 in the world rankings.
WTA Finals
Six first-timers, but whose run to WTA Finals is most surprising?
10/11/2021 AT 06:42

Simona Halep of Romania congratulates Kiki Bertens of Netherlands after their match during the women's final of the Western & Southern Open at Lindner Family Tennis Center on August 19, 2018 in Mason, Ohio.

Image credit: Getty Images

It also ended a nine-match winning streak by Halep, who lost in the Cincinnati final for a third time.
"Maybe once I will win one because I love to play here," said Halep. "I was a little bit tired this week but it was a pleasure to come here and play.
"Definitely, I want to congratulate Kiki. You played amazing and you definitely deserve this title."
WTA Cluj-Napoca
Kontaveit storms past Halep to win Transylvania Open final and book ticket to WTA Finals
31/10/2021 AT 17:32
WTA Cluj-Napoca
Halep breezes past Kostyuk to set up Transylvania Open final against Kontaveit
30/10/2021 AT 15:48