Seventh-seeded Magdalena Rybarikova of Slovakia beat unseeded Andrea Petkovic of Germany 6-4, 7-6 (2) on Sunday to win the Citi Open for the second consecutive year.
Rybarikova, who is ranked 43rd, improved her record at the hard-court tournament to 10-0.
Half of her four career titles have come in Washington.
Petkovic is a former top-10 player who is coming back from a series of injuries and is currently ranked 64th. She had a tough turnaround, coming back to play in the final a little more than 15 hours after her rain-delayed semifinal finished past 2 a.m. Sunday
Earlier Sunday, top-seeded Juan Martin del Potro beat eighth-seeded John Isner 3-6, 6-1, 6-2 in the men's final.