The 23-year-old American will face Russian qualifier Elena Vesnina, 29, who ousted fifth-seeded Sara Errani of Italy 6-4 4-6 6-2 in a battle that stretched for two hours and 20 minutes.
Kerber was attended to by trainers after losing her serve to the American to fall behind 2-0 in the second set, but gave it another try before deciding to retire.
Stephens, who has won titles this year at Auckland and Acapulco, was in total command of her game, hitting 14 winners while committing only four unforced errors in the match and did not face a single break point.
"I think I executed very well," the 25th-ranked Stephens told ESPN.
"The wind was crazy today. I shanked on the first shot because it was going everywhere. I was happy I was able to keep the ball in the court."
Kerber had much more trouble handling the conditions and Stephen's deliveries as she made 16 unforced errors in the 10 games.
Vesnina, who won the mixed doubles title at the Australian Open with Brazilian Bruno Soares, made the critical break in the sixth game of the third set for a 4-2 lead and broke again to secure her place in Sunday's final.
(Reporting by Larry Fine in New York; Editing by Andrew Both)
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