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WASHINGTON >> Australian Open champion Caroline Wozniacki pulled out of the Citi Open because of an injured right leg today, when the hard-court tournament also lost its defending champion, Ekaterina Makarova.

The No. 4-seeded Makarova lost her first-round match at the U.S. Open tuneup to Ana Bogdan 7-6 (2), 6-3.

No. 2 seed Sloane Stephens, the reigning U.S. Open champion, moved into the second round with a 7-5, 6-4 victory over Bethanie Mattek-Sands. Stephens converted all four break points she earned, saved seven of the nine she faced, and hit six aces. She’ll next face Andrea Petkovic, who won her opener on Monday.

In other results Tuesday, No. 3 Naomi Osaka beat Bernarda Pera 6-2, 7-6 (4), and No. 7 Donna Vekic defeated Caroline Dolehide 6-3, 6-4.

No. 1 seed Wozniacki’s withdrew hours before she was to face qualifier Anhelina Kalinina of Ukraine. Wozniacki was replaced in the field by Ysaline Bonaventure, who lost in qualifying.

After practicing on-site Monday, Wozniacki said her leg is “just something that has been nagging a little bit, but hopefully it’ll be fine.”

She won her first Grand Slam title in January at Melbourne to return to No. 1 in the rankings. She is currently No. 2.

Wozniacki hasn’t played a match since losing in the second round at Wimbledon on July 4.

She now has less than a month to get ready for the U.S. Open, which starts Aug. 27.