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 seasons, the big-serving Isner sent down 13 aces in his 7-5 6-4 win.

He 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 stark contract to his tough three-set second round victory over Australian John Millman, Ryan Harrison trounced wildcard Christopher Eubanks to progress to the final four.

The fourth-seeded Harrison won the all-American quarter-final 6-1 6-2 in just 55 minutes.

Jack Sock, the top seed, faces fifth-seeded Briton Kyle Edmund in Friday night’s quarter-final.