Miami: John Isner's magical ATP Miami Open run continued Wednesday when the big-serving American overpowered South Korean Chung Hyeon 6-1 6-4 to book his place in the semi-finals.

Isner, the 14th seed, arrived in South Florida having won just two matches all year but his fourth victory of the week was enough to put him within touching distance of his first ATP Masters title.

The big-serving 32-year-old Americans fired 12 aces to dispatch 19th-seeded Chung in only 68 minutes.

Chung, playing in his sixth last-eight match of the year, defeated Isner in their previous meeting in Auckland but was unable to stay in the match after being steamrolled in an opening set that saw him broken for the first time this tournament.

"I played extremely well and every match I have gotten better and stronger," Isner said.

"The conditions were perfect for me. The way the ball bounces, the court isn't too fast, so it's great for me. I have played well here before but I am delighted to get this far."

Isner will next face either Argentina's Juan Martin del Potro or Canada's Milos Raonic, who play in a later quarter-final.