Sock set up a quarterfinal match against fifth-seeded Kyle Edmund of England, a 2-6, 6-4, 7-5 winner over Germany's Peter Gojowczyk 2-6, 6-4, 7-5 in the hard-court event.
The 24-year-old Sock has three career victories, winning this year in Auckland, New Zealand, and Delray Beach, Florida.
Fourth-seeded Ryan Harrison also advanced at Atlantic Station, beating Australia's John Millman 6-7 (2), 7-6 (4), 7-5. The American will face Christopher Eubanks, a 6-4, 4-6, 6-2 winner over eighth-seeded Jared Donaldson in an all-American match.
In the other quarterfinals, second-seeded John Isner will play Slovakia's Lukas Lacko, and third-seeded Gilles Muller of Luxembourg will face American Tommy Paul. Isner is seeking his fourth Atlanta title in five seasons. He won the Hall of Fame Open on Sunday in Newport, Rhode Island.