Malek Jaziri saved an incredible eight match points to win through at the Atlanta Open, while Kyle Edmund also advanced.
Jaziri battled past wildcard Reilly Opelka 5-7 7-6 (16-14) 6-1 in the first round at the ATP 250 event on Tuesday.
The Tunisian saved four match points in the 12th game of the second set and four more in a marathon tie-break.
Edmund, the British fifth seed, had it far easier, beating 2015 runner-up Marcos Baghdatis 6-3 7-5.
The only other seed in action was Donald Young, who cruised past qualifier Tim Smyczek 6-4 6-2.
Dudi Sela, Peter Gojowczyk, John Millman, Christopher Eubanks and Vasek Pospisil also advanced.