Roberto Bautista Agut booked his spot in the Winston-Salem Open semi-finals, while Steve Johnson was beaten on Thursday.
Bautista Agut, the top seed at the ATP 250 event, overcame American teenager Taylor Fritz 6-2 7-6 (7-3) in their quarter-final.
Last year's runner-up, Bautista Agut only lost 11 points on serve in an impressive display.
Awaiting the Spaniard in the last four is Jan-Lennard Struff after the German claimed a 6-4 7-6 (7-2) win over Borna Coric.
On the other side of the draw, seeds Johnson and Chung Hyeon were beaten.
Qualifier Kyle Edmund got past Johnson 5-7 6-3 6-3 and the Brit will meet Damir Dzumhur, who was too good for Chung 6-4 6-2.