Houston (AFP) - Top seed and 2015 winner Jack Sock defeated Germany's Tommy Haas 6-4, 3-6, 6-3 to reach the quarter-finals of the US Clay Court Championships on Thursday.

Sock will next face sixth-seeded Feliciano Lopez of Spain, who beat Chung Hyeon of South Korea 6-4, 2-6, 6-1.

Sock is seeking his third title of the year to go with victories at Delray Beach, Florida and Auckland, New Zealand.

He is one of four seeded Americans left in the tournament's top five.

Second seed John Isner survived an upset bid from qualifier Leonardo Mayer of Argentina with a 6-7 (6-8), 7-6 (7-3), 6-3 win in a contest that lasted over two hours.

Isner will face wildcard Ernesto Escobedo in the quarters.

Sam Querrey, the third seed, cruised to a 6-4, 6-4 victory over Argentinian Horacio Zeballos to reach the quarters for the third-straight year.

Fourth-seeded Steve Johnson also won in straight sets over Dustin Brown of Germany, but the American had to survive a marathon opening set before advancing with a 7-6 (14-12), 6-4 win.

No. 5 seed Fernando Verdasco advanced to a match-up against Johnson with a 7-6 (7-4), 6-1 win over Argentina's Nicolas Kicker.