The 33-year-old, who lives in Houston, was dominant in every department, winning 70 per cent of points on first serve and breaking his compatriot five times to claim his first-ever win over a top-10 player.

He will now face fellow American Ryan Harrison after the teenager beat Russia's Igor Kunitsyn 6-2 6-4.

Second seed John Isner avoided an upset by battling back to secure a 6-7 (3/7) 6-4 6-2 win over Horacio Zeballos.

That sent Isner through to an all-American quarter-final against defending champion Ryan Sweeting, who beat compatriot Bobby Reynolds 6-3 6-2.

Fourth seed Juan Monaco is through after a routine 6-3 6-3 win over Tatsuma Ito, however fifth-ranked Kevin Anderson had to dig deep before overcoming Sam Querrey 7-6 (7-3) 3-6 7-6 (7-2).