The seventh-seeded Rybarikova climbed out of an 0-4 hole in the first set and went on to post a 6-4, 7-6 (7-2) victory over Petkovic at FitzGerald Tennis Center.
The 24-year-old triumphed over Anastasia Pavlyuchenkova in last year's Washington finale.
Petkovic, meanwhile, started fast but ultimately came up short in her bid for a third career title.
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