John Isner's quest for a fourth successive Atlanta Open remains on track, while second seed Nick Kyrgios also reached the semi-finals on Friday.
Three-time defending champion Isner triumphed 7-5 6-4 over fellow American and eighth seed Taylor Fritz in the quarter-finals.
In a trademark display, top seed and big-serving American Isner fired down 20 aces - 12 of those coming in the opening set - while he converted three of eight break points to set up the straight-sets win and a final-four berth.
It will be a battle of the giants after wild card Reilly Opelka advanced to his first ATP Tour semi-final.
Opelka, who measures at 2.11metres tall and is the tallest player on tour alongside Ivo Karlovic, was too good for American counterpart and seventh seed Donald Young in sweltering conditions.
The emerging 18-year-old accounted for Young 6-4 6-4 after 71 minutes on court.
Australian second seed Nick Kyrgios will contest his fifth semi-final of the year thanks to his three-set victory over Fernando Verdasco.
Kyrgios outlasted Verdasco 6-4 6-7 (5-7) 6-3 in Atlanta.
After winning the opening set having dropped just four points on serve, the second was tougher for Kyrgios, who saw his 4-2 lead in the tie-break evaporate as Verdasco won five of the final six points.
Kyrgios, however, was not to be denied after dropping just five points on serve in the deciding set.
Next up for Kyrgios is Yoshihito Nishioka, who saw off Horacio Zeballos 6-4 6-4.