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Houston - Top seed and former 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 in three sets 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.
The 2015 champ in this event, Sock is one of four Americans seeded in the tournament's top five.
Number-two 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 Argentine 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 matchup against Johnson with a 7-6 (7/4), 6-1 win over Argentina's Nicolas Kicker.
Results from ATP Houston tournament on Thursday:
Second round
Fernando Verdasco (ESP x5) bt Nicolas Kicker (ARG) 7-6 (7/4), 6-1
John Isner (USA x2) bt Leonardo Mayer (ARG) 6-7 (5/7), 7-6 (7/3), 6-3
Jack Sock (USA) bt Tommy Haas (GER) 6-4, 3-6, 6-3
Steve Johnson (USA x4) bt Dustin Brown (GER) 7-6 (14/12), 6-4
Thomaz Belluci (BRA x8) bt Maximo Gonzalez (ARG) 2-6, 7-6 (7/5), 6-2
Sam Querrey (USA x3) bt Horacio Zeballos (ARG) 6-4, 6-4
Ernesto Escobedo (USA) bt Thiago Monteiro (BRA) 4-6, 6-4, 6-4
Feliciano Lopez (ESP x6) bt Chung Hyeon (KOR) 4-6, 6-2, 6-1
