Houston: Germany's Dustin Brown upset top seeded American John Isner in three sets to reach the quarter-finals of the US men's clay court championships on Wednesday.

The world number 101 Brown outlasted the hard-hitting defending champ Isner 6-4, 6-7 (7/9), 7-6 (7/4) on the clay courts at the River Oaks Country Club.

Isner forced a deciding set by winning an extended tiebreaker in the second, but Brown rebounded to claim the final tiebreak. 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 quarter-finals after the 2010 Houston finalist toppled former champ Lleyton Hewitt of Australia earlier on Wednesday.

Querrey beat seventh-seeded former world number one Hewitt 6-3, 6-4 in just 76 minutes. Hewitt, who is enjoying a resurgence this year, won in Houston back in 2009.

In opening-round play on Wednesday, Argentine Juan Monaco stopped India's Somdev Devvarman 7-6 (7/4), 6-2 and Colombian Santiago Giraldo ousted Aussie Matthew Ebden 6-1, 6-2. Monaco titled here two years ago.