World number 19 John Isner's Atlanta three-peat remains on course, after he won his first match at the 2015 Atlanta Open.
John Isner survived the seed cull at the Atlanta Open, which saw Jack Sock, Steve Johnson and Benjamin Becker defeated.
Top seed Isner trailed by a break in each set to Radek Stepanek, but eked out a 7-6 (7-3) 7-6 (9-7) victory at the ATP 250 hard-court event.
Isner is aiming for an Atlanta three-peat, and has made the final in Georgia in four of the past five years - and will play Ricardas Berankis in the quarter-finals.
Berankis ousted sixth seed Johnson 6-4 6-2, the Lithuanian sending down 10 aces in the one-hour, 11-minute victory.
Qualifier Denis Kudla and Dudi Sela also set up an unlikely quarter-final match-up.
American Kudla downed third seed Sock 7-6 (8-6) 6-3, twice coming from a break of serve down in the first set - and also saving a set point in the tie-break.
Isner and Kudla are the only American hopes left in the field, with locals having won all five of the previous editions in Atlanta - and also five of the seven prior to that when the tournament was hosted in Indianapolis.
Finalist in Atlanta in 2014, Sela defeated eighth-seeded veteran Becker 7-5 6-3, in a re-match of their semi-final last year.