Serena Williams rallied to advance to the Western & Southern Open final, beating Caroline Wozniacki 2-6, 6-2, 6-4 on Saturday.
The top-ranked Williams lost a set for the first time in the tournament and committed 41 unforced errors, 20 more than Wozniacki.
Williams will face ninth-seeded Ana Ivanovic in the final. Ana Ivanovic beat fifth-seeded Maria Sharapova 6-2, 5-7, 7-5 in a night match.
"It was definitely feeling super tight in the match," she said. "That's when I really relaxed, to be honest. By then I was able just to go for more shots and come to the net more and just kind of just not have anything to lose."
She also tried to shorten points with booming serves, leading to getting just 58 percent of her serves in play.
"I just had to go for rockets because I wasn't feeling great," she said. "I thought, 'Listen, I'm going to go out and just try to hit aces and see what happens.' It started working for me. I was like, 'OK.' Hopefully my arm will be OK tomorrow. We'll see."
Williams and Wozniacki both struggled with their serves, leading to a combined 15 service breaks, including the first five games of the third set. Wozniacki was broken in eight of nine service games in one stretch and connected on just 54 percent of her first serves.
"I didn't get many first serves in," Wozniacki said. "When I did, they weren't placed very well. It's frustrating, because you lose a set 6-4 in the third and you only hold serve once.
"I actually broke Serena three times in the set. I don't think that happens very often to her either, so, you know, you feel like you're there and my all game was there. I was fighting. I was running. I was trying to take the ball early. I was returning well. Then my serve comes around and I can't seem to hold serve. It's frustrating thing when you're out there on the court."
Williams has never won the Cincinnati-area tournament in five previous appearances. She lost a third-set tiebreaker to Victoria Azarenka last year.
With the win, Williams also wrapped up the U.S. Open Series women's title. Williams has won it three of the past four years and will attempt to set another record for the largest payout in tennis history at the U.S. Open - $4 million ($3 million for winning the U.S. Open and a $1 million bonus for winning the U.S. Open as the series champion).
The two-time defending U.S. Open champion won both the U.S. Open Series title and the U.S. Open last year. She is the only woman to win the U.S. Open Series bonus challenge three times (2011, 2013, 2014). No other woman has won it more than once.