Top-seeded Jack Sock outlasted Germany's Tommy Haas 6-4 3-6 6-3 to lead five Americans into the US Men's Clay Court Championship quarter-finals.
Second-seeded John Isner, third-seeded Sam Querrey, fourth-seeded Steve Johnson and unseeded Ernesto Escobedo joined their US compatriot in the final eight at River Oaks.
Sock won the tournament in 2015 for his first ATP Tour title and added victories this year in Auckland, New Zealand, and Delray Beach, Florida.
He will play sixth-seeded Feliciano Lopez of Spain, a 4-6 6-2 6-1 winner over South Korea's Chung Hyeon.
Isner topped Argentina's Leonardo Mayer 6-7 (7-5) 7-6 (7-3) 6-3.
The 2013 winner will face Escobedo, a 4-6 6-4 6-4 winner over Brazil's Thiago Monteiro.
Johnson will play fifth-seeded Fernando Verdasco of Spain.
Johnson edged Germany's Dustin Brown 7-6 (14-12) 6-4 and Verdasco beat Argentina's Nicolas Kicker 7-6 (7-4) 6-1.
Querrey set up a match against eighth-seeded Thomaz Bellucci of Brazil.
Querrey beat Argentina's Horacio Zeballos 6-4, 6-4 and Bellucci outlasted Argentina's Maximo Gonzalez 2-6 7-6 (7-5) 6-2.
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