American John Isner, the second seed, has beaten Slovakia's Lukas Lacko in straight sets to advance to the Atlanta Open semi-finals.
Seeking his fourth Atlanta title in five years, the big-serving Isner sent down 13 aces in his 7-5 6-4 win.
The 2016 runner-up to Australian Nick Kyrgios will face Gilles Muller of Luxembourg after the third-seed served 16 aces in his routine 6-3 6-1 victory over American qualifier Tommy Paul.
In Friday night's quarter-final, Jack Sock had his serve broken four times in his defeat to fifth-seeded Kyle Edmund.
The Briton took less than 90 minutes to record a 6-4 6-1 win against the top-seeded American.
In stark contract to his tough three-set second round victory over Australian John Millman, Ryan Harrison trounced wildcard Christopher Eubanks.
The fourth-seeded Harrison won the all-American quarter-final 6-1 6-2 in just 55 minutes to progress to a final-four meeting with Edmund.
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