Third-seeded Petkovic brought a perfect 11-0 record on the green clay courts of Charleston into the meeting with her fifth-seeded countrywoman.
Petkovic seized an early break for a 2-0 lead, but Kerber won 11 of the next 14 games en route to a 6-4, 6-4 triumph.
Trailing 6-4, 5-1, Petkovic didn’t go down without a final fight.
She won three games on the trot to narrow the gap to 5-4 in the second set before Kerber served out the match with a love game.
“I’m so close with Andrea. We’re really good friends. It’s always tough to play against someone like that,” Petkovic said. “But I was just trying to focus on myself and my game.
“But also congratulations to Andrea – she’s back in the top, and I wish her all the best.”
Kerber is through to her first WTA final since a runner-up finish to Serena Williams at Stanford last year.
In Keys she’ll face a rising talent eager to add a second WTA title to her resume.
Her first came at the expense of Kerber at Eastbourne last year in Keys’s first WTA final.
The seventh-seeded American is assured of reaching a new career high ranking of 17th in the world after beating Czech Lucie Hradecka 6-1, 6-4 in the semi-finals.
Hradecka had come out of qualifying and beaten three seeds en rout to the final four: 11th-seeded Zarina Diyas, number eight Caroline Garcia and fourth-seeded Sara Errani.
However, she couldn’t get to Keys, who saved the only break point she faced in the match and broke the Czech three times to finish off the victory in 67 minutes.
“I was just going out and trying to focus on my game, and I knew what I was trying to do,” Keys said.
“It’s always great when you can get off to a good start, and after that I just kept going with it.”