An all-American semi-final is in store at the US Men's Clay Court Championship in Houston after round of 16 wins for John Isner, Jack Sock, Steve Johnson and Taylor Fritz.
Top seed Isner breezed past Switzerland's Henri Laaksonen, winning 6-4 6-2 in a little over an hour.
Sock, the third seed, dispatched Argentina's Horacio Zeballos 6-4 6-4.
It was also smooth-running for sixth-seeded Johnson, who beat compatriot Frances Tiafoe 6-3 6-4.
Fritz produced the one upset of the day, knocking out seventh-seeded compatriot Ryan Harrison in three sets.
The world No.72 rallied after losing the first set to triumph 3-6 6-0 6-3.
His reward was a quarter-final showdown with Sock while Isner will play Johnson.
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