Jack Sock and John Isner came through testing second-round matches to reach the US Men's Clay Court Championship quarter-finals.

Sock, the top seed at the ATP 250 event in Houston, overcame German veteran Tommy Haas 6-4 3-6 6-3.

The American is enjoying an excellent 2017 and moved into his fifth quarter-final of the year with victory in just under two hours.

Awaiting Sock – the 2015 champion and last year's runner-up – in the last eight is Feliciano Lopez, who got past Chung Hyeon 4-6 6-2 6-1.

Isner progressed with a hard-fought 6-7 (5-7) 7-6 (7-3) 6-3 win over Argentine qualifier Leonardo Mayer.

The American second seed, winner in 2013, sent down 20 aces to reach the quarter-finals.

Isner will face Ernesto Escobedo, the American wildcard having beaten Brazilian Thiago Monteiro 4-6 6-4 6-4.

The seeds enjoyed a good day as Steve Johnson, Fernando Verdasco, Sam Querrey and Thomaz Bellucci also won through.

Johnson edged past Dustin Brown 7-6 (14-12) 6-4 to set up a clash against Verdasco, who was too good for Nicolas Kicker 7-6 (7-4) 6-1.

Querrey, a two-time runner-up, beat Horacio Zeballos 6-4 6-4 and next faces Bellucci after the Brazilian had a 2-6 7-6 (7-5) 6-2 win over Maximo Gonzalez.