Sandgren was playing for the first time since the Australian Open. He has lost both matches since winning the ASB Classic in January for his first ATP title.
Also, Australian Bernard Tomic beat Lukas Lacko of Slovakia, 6-4, 6-2. He will next face top-seeded John Isner of the U.S.
Isner became the highest-ranked player in the second-year tournament at the Nassau Memorial Veterans Coliseum when defending champion Kevin Anderson had to withdraw due to a right elbow injury.
Seventh-seeded Jordan Thompson of Australia beat American Noah Rubin 6-7 (7), 7-5, 6-1.
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