Sixth seed Sam Querrey had to come from a set down to beat Guillermo Garcia-Lopez in the second round of the Winston-Salem Open.
Querrey was forced to dig deep to win 4-6 6-3 6-4 in one of three second-round matches in North Carolina.
The two others both saw seeded players crash out, with Robin Haase upsetting 12th-ranked Lukas Rosol 6-4 3-6 7-5, and Yen-Hsun Lu dispatching 16th seed Pablo Andujar 6-4 6-3.
In first-round action, there were wins for qualifiers David Goffin, Steve Johnson and Thiemo De Bakker, who progressed alongside Ricardas Berankis, Roberto Bautistaagut, James Blake, Tim Smyczek, Guido Pella, Guillame Rufin, Nicolas Mahut and Daniel Gimeno-Traver.
However two-time defending champion John Isner and top seed Tomas Berdych have both pulled out of the event due to injuries.
"It is really unfortunate I have to withdraw from the Winston-Salem Open, one of my favourite tournaments of the year," Isner told the official ATP website.
"I've had so much success playing in front of great crowds as well as my friends and family.
"This was a really tough decision but with injuries taking me out of the Australian Open and Wimbledon, I need to fully heal prior to the US Open."
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