Top seed Barty, who would have replaced the injured Naomi Osaka as the world number one with a victory, struggled with the rhythm off her serve as she lost 6-2 6-4, losing eight consecutive games to the Russian at one point.
“I think it probably started with the serve for me,” Barty said courtside. “I mean, giving Svetlana that many looks at second serves I know I’m not going to get free points off my first serve off Sveta. She’s an incredible returner and retrieves a long way so is able to make a lot of returns.
“Just to be on the back foot of a lot of rallies from the start wasn’t ideal. I felt like we have done well to fight all week and then to put ourselves in this position, but Sveta was unreal today.”
Kuznetsova, ranked world number 153, will play American Madison Keys for the title on Sunday.
Keys, the 16th seed, turned back compatriot Sofia Kenin 7-5 6-4 in their semi-final.
Winner of the U.S. Open in 2004 and the French Open in 2009, the 34-year-old Kuznetsova has struggled with injuries throughout her career but has now defeated eighth seeded Sloane Stephens, Czech third seed Karolina Pliskova and Barty in successive matches.
“It’s amazing to play again on center court,” Kuznetsova said.
She entered the Cincinnati tournament as a wild card after winning two matches in Toronto. Issues in obtaining a U.S. visa kept her from defending her Washington DC title.
“Maybe one week or two weeks helped me back home,” she said. “Something important happened in my life and now I feel better.”
Barty won the first two games, then lost the set as Kuznetsova, handling the Australian’s slices effectively and serving well, won the next six.
Barty at one point called a medical time-out to have her right ankle retaped.
But the streak continued in the second set, with Kuznetsova winning the opening two games before Barty held.
The Australian rallied to 4-3 and 5-4 before Kuznetsova served out the victory.
Keys had 41 winners, fired 14 aces and won 80% of her first serves to make her first hardcourt final in two years.
The 2017 U.S. Open finalist raced to a 5-2 lead in the first set before Kenin saved a set point and won three games in a row to level things at 5-5.
She fell a break down twice in the second set, but rallied to win the last three games to claim the match.