FILE PHOTO: Jul 2, 2018; London, United Kingdom; Sloane Stephens (USA) in action during her match against Donna Vekic (CRO) on day one at the All England Lawn and Croquet Club. Mandatory Credit: Susan Mullane-USA TODAY Sports

(Reuters) - Sloane Stephens began the build-up to her U.S. Open title defense with a 7-5 6-4 win over wildcard Bethanie Mattek-Sands back on the American hardcourts at the Citi Open in Washington, DC on Tuesday.

Stephens, who won her first career title in Washington in 2015, finished off Sands with her sixth ace of the match to book a second round meeting with Germany’s Andrea Petkovic, who was a 6-1 6-1 winner over American Jamie Loeb.

It marked the fifth time in six meetings that Stephens has beaten fellow American Sands, all of the five other matches having been played on clay.

“I’m super excited to be back on hard courts again. I love running around out there,” Stephens said after the match.

“I felt super comfortable, which was good,” she said.

“Although the ball was coming really hard and she was crushing and rushing, I was ready.”

The third ranked Stephens is the top remaining seed in the draw after world number two Caroline Wozniacki withdrew earlier on Tuesday with a right leg injury.

Elsewhere third-seeded Naomi Osaka won her debut match at the Citi Open with a 6-2 7-6(4) over American Bernarda Pera while eighth-seeded Donna Vekic broke Caroline Dolehide three times en route to a 6-3 6-4 victory.

Romanian Ana Bogdan stunned fourth seed Ekaterina Makarova 7-6(2) 6-3 and Kazakh Yulia Putintseva outlasted German Tatjana Maria 6-3 3-6 6-4.