Isner, the world No 10 and second seed, fired down 11 aces and saved all six break points that he faced in the all-American quarter-final.

The 26-year-old, who is seeking his first clay-court title, now faces Spanish third seed Feliciano Lopez, who led 6-4 2-2 when Carlos Berlocq of Argentina retired injured.

The other semi-final will see Argentinian fourth seed Juan Monaco, who edged out South Africa's world No 33 Kevin Anderson 7-6 (7-4) 7-5, take on US qualifier Michael Russell.

Russell outlasted Ryan Harrison 6-1 5-7 6-3 in another all-American last-eight clash.

The 33-year-old delighted his hometown crowd by reaching his first career ATP semi-final by holding off 19-year-old Harrison.