Germany's Dustin Brown upset top-seeded American John Isner in three sets to reach the quarterfinals of the US men's clay-court championships on Wednesday (US time).

World No.101 Brown outlasted the hard-hitting defending champion Isner 6-4, 6-7 (7/9), 7-6 (7/4) on the clay courts at the River Oaks Country Club in Houston, Texas.

Isner forced a deciding set by winning an extended tiebreaker in the second, but Brown rebounded to claim the final tiebreaker. It was his first-ever victory over a top-10 opponent.

Isner, who is ranked ninth in the world, beat Nicolas Almargo in the 2013 final.

Brown will next face Sam Querrey in the quarterfinals after the 2010 Houston finalist toppled former champion Lleyton Hewitt of Australia 6-3, 6-4 earlier Wednesday.