Gauff, the 15-year-old who reached the fourth round at Wimbledon and got into the Citi Open as a qualifier, lost 6-4, 6-2 to Zarina Diyas. Stephens is ranked eighth and Peterson is 70th. Stephens won the Citi Open in 2015 and reached the 2017 doubles final. Peterson, from Sweden, will face Italian Camila Giorgi in the second round. Giorgi posted a 6-2, 7-5 win over Sachia Vickery of the United States.
In the men's draw, Jo-Wilfried Tsonga outlasted second-seeded Karen Khachanov for a 6-4, 2-6, 7-5 victory. Daniil Medvedev, Marin Cilic, Frances Tiafoe and Kyle Edmund also advanced. In other action in Washington, eighth-seeded Monica Puig of Puerto Rico defeated American Allie Kiick 6-4, 6-2.
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