The fourth seed was stretched in a 3-6, 7-6(4), 6-4 victory over Lukas Lacko. The Slovak proved a tough nut to crack for Sock, with Lacko saving all but one break point in the two hour contest.
Sock will face Jan Hernych in the quarter-finals after the Czech saved a match point in his clash with Colombian Alejandro Falla before claiming a 7-6(5), 6-7(6), 7-6(7) victory.
Second seed Ivo Karlovic had a much easier time of it in his 6-4, 7-6(4) win over Malek Jaziri.
Having beaten top seed John Isner in the first round, Rajeev Ram followed up that strong showing with a 6-4, 6-3 win over Japan’s Yuichi Sugita.
Next on the agenda for Ram is a clash with fifth seed Adrian Mannarino The Frenchman had a busy day on Wednesday, as he first had to complete his first round encounter with Tommy Haas, a match he won in three sets, before defeating compatriot Edouard Roger-Vasselin 1-6, 6-3, 6-2.
Local hope Jared Donaldson saw his title challenge ended by John-Patrick Smith, with the Australian claiming a 6-3, 6-1 victory.
Smith will come up against Tatsuma Ito in the last eight after the Japanese player downed seventh seed Steve Johnson 6-4, 6-4.
Sixth seed Sam Querrey's second round clash with Dustin Brown, who beat Rafael Nadal at Wimbledon, will go into a second day after rain brought a halt to proceedings with Brown leading 6-2, 4-6, 4-3.