Madison Keys overpowered Caroline Wozniacki 7-6 (5), 6-3 to win the Charleston Open and defeat the former world number one for the first time on Sunday.
The win, her first on clay court, broke a nearly two-year drought for the eighth-seeded American, who claimed her fourth career title.
Her previous two losses to Denmark's Wozniacki had been on hard courts when Keys failed to win a set.
"I think I served really well today, said Keys, who had predicted on Saturday that playing on clay would be beneficial in her bid to defeat Wozniacki.
"I also felt I kept a lot of pressure on her, making a lot of returns and keeping rallies going and not going for things too early," said the world number 18, who won 76 per cent of points on her first serve, and fired six aces.
That was most evident late in the first set and the tiebreak.
The big-serving Keys trailed 5-6 in the opening set before forcing the tiebreak, which remained close until 4-3 when the fifth-seeded Wozniacki double-faulted.
Keys won the next point but twice had backhand errors to allow Wozniacki to pull within 6-5.
A backhand down the line enabled Keys to finally claim the set.
"I just knew the games I was losing in the first set I was just being too tentative and not really trusting my shot," said the 24-year-old American.
"I knew I had to go for it. I think that's how I have to play... 100 percent believing in my game."
Keys broke Wozniacki in the sixth game of the second set to go up 4-2 and when she made it 5-2 that all but clinched the match.
"It's definitely been a really great week for me, especially after having some really tough losses the last couple of weeks," Keys said. "It definitely is a very good springboard for heading over to Europe."
Spain's Garbine Muguruza successfully defended her Monterrey Open title on Sunday when Victoria Azarenka retired injured from her first WTA singles final in three years.
Second seed Muguruza was leading the match between the two former world number ones 6-1, 3-1 when Azarenka decided she was unable to continue because of an injury to her right leg.
"It's unfortunate I couldn't give my best today, but I tried my hardest," a tearful Azarenka, who had not previously reached final since the birth of her first child in December 2016, said at the presentation.