Third seed John Isner fell to Croatian Borna Coric 7-5, 6-2 in the Winston-Salem Open last 16 as the quarterfinalists were confirmed in North Carolina.
Isner was broken in the first -- with the set tied at 5-5 -- before Coric comfortably clinched the second to set up a meeting with Jan-Lennard Struff.
The German needed a third-set tiebreak to dismiss the challenge of Andreas Seppi as he won 6-3, 6-7, 7-6.
Taylor Fritz, 19, set up a meeting with top seed Roberto Bautista Agut after edging Paolo Lorenzi 7-6, 7-6.
Agut eased into the quarters with a 6-3, 6-3 win over Marcos Baghdatis.
Having overcome Carlos Berlocq 7-6, 6-4 -- Steve Johnson faces British No. 2 Kyle Edmund who cruised into quarterfinals by crushing Hungarian opponent Marton Fucsovics.
Edmund, ranked No. 45 in the world, was untroubled as he posted a 6-2, 6-1 win.
Having upset Pablo Carreno Busta in the second round, Frenchman Julien Benneteau was forced to retire against Korean Hyeon Chung.
After losing the opening set 6-4, Benneteau retired with the second set level at a game apiece.
Chung will now face Damir Dzumhur in the last eight after the Bosnian defeated Horacio Zeballos 7-6, 6-3.
The Press Association contributed to this report.