The men's draw suffered a minor blow when John Isner, who won the Atlanta title last week and has been beaten three times in the final in Washington in 2007, 2013 and 2015, had to withdraw from the tournament with a knee injury.
"Disappointed to have withdrawn from the @CitiOpen this year," Isner posted on Twitter. "Unfortunately, I'm just not quite fit enough for it this year. I'll be back!"
In other first-round men's action, American Donald Young beat compatriot Tim Smyczek 4-6 6-3 6-4, to set up a second round clash with world number nine Kei Nishikori.
World number seven Dominic Thiem, the top seed, is due to play on Tuesday, while former Argentina's U.S. Open champion Juan Martin del Potro is also on court.
Women's third seed Lauren Davis was upset by Aryna Sabalenka 7-5 6-3 on Monday, while former number one Jelena Jankovic retired while trailing Oceane Dodin 2-0 in the third set.
German fourth seed Julia Goerges defeated Francoise Abanda 6-4 6-4.
Top seeded Simona Halep will face American Sloane Stephens, who won the tournament in 2015 on Tuesday.
(Writing by Jahmal Corner in Los Angeles)
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