Argentina's fourth-seeded Juan Monaco outlasted US second seed John Insner for a 6-2, 3-6, 6-3 triumph on Sunday in the final of the $442,500 ATP US Men's Clay Court Championship.
Monaco, ranked 16th in the world, captured his fifth career ATP title and his second of the year after a February crown at Vina del Mar. Before that, he had not won an ATP title since 2007.
Monaco, who improved to 5-8 in ATP finals, had reached the semi-finals last month at Miami, losing to the eventual winner, world number one Novak Djokovic.
Isner saved 12 of 15 break points that he faced, but lost the only one he saw in the third set to surrender a break in the penultimate game of the match. Monaco held once more to claim the crown after two hours and 27 minutes.
Despite the defeat, Isner will become the top-ranked American player in the world on Monday when he passes countryman Mardy Fish and moves up one spot into ninth in the ATP rankings.
Isner, who sparked a US semi-final Davis Cup triumph on French clay last week to book a semi-final date with Spain in September, missed a chance for his fourth career ATP title and the first on clay.
Isner, a runner-up to Roger Federer last month at Indian Wells, lost to fellow American Sam Querrey in the 2010 Belgrade final in his only prior clay final.
Monaco had lost both prior matches against Isner, including a third-round affair last month at Indian Wells.