World No.10 John Isner beat defending champion Ryan Sweeting 7-6 (7-1) 7-6 (7-4) on Friday to reach the semi-finals of the $US442,500 ($A425,350) ATP US Men’s Clay Court Championship.

Isner, the second seed, fired 11 aces and saved all six break points that he faced in the all-American quarter-final to improve to 19-6 for the season and keep alive his hopes for a first clay-court title.

Isner, a runner-up last month at Indian Wells, will next face Spanish third seed Feliciano Lopez, who led 6-4 2-2 when Carlos Berlocq of Argentina retired injured.

Lopez is chasing his third ATP title and his first on clay.

The other semi-final will see Argentine fourth seed Juan Monaco, who edged South Africa’s Kevin Anderson 7-6 (7-4) 7-5, take on US qualifier Michael Russell, who outlasted Ryan Harrison 6-1 5-7 6-3 in another all-American quarter-final.

Monaco, ranked 16th, won his fourth career ATP title, and first since 2007, in February at Vina del Mar and reached the semi-finals last month at Miami, losing to eventual winner, world No.1 Novak Djokovic.

Anderson, ranked 33rd, was denied a chance for his second title in as many months after taking his second career ATP crown last month at Delray Beach, Florida.

Russell, a 33-year-old hometown hero, reached his first career ATP semi-final by holding off 19-year-old Harrison.