Jack Sock Enters Quarter-Finals of ATP Houston Open

Defending champion and fourth seed Jack Sock held off Australian qualifier Matthew Barton 6-2, 7-6 (7/5) on Thursday to reach the quarter-finals of the ATP claycourt tournament in Houston, Texas. (Latest Tennis News)

Sock's serve was rock solid, with the American winning 80 percent of his first serve points to wrap up the contest in 75 minutes.

"It's never easy playing a guy you've never faced before and who you haven't seen," Sock said. "He's very athletic and has a pretty big serve. He hit a couple (of serves) at 138 mph. I didn't realise he had that massive of a serve."

Sock, who had a first-round bye, was glad to finally open his account at a tournament where he won his first ATP title last year as an unseeded player.

Now ranked 25th in the world, Sock is trying to reach his second final of 2016, after retiring because of illness against Roberto Bautista Agut in Auckland in January.

Sock faces a tough quarter-final against seventh-seeded Cypriot Marcos Baghdatis, who downed Spain's Fernando Verdasco 6-4, 6-3.

Wild card Tim Smyczek needed less than an hour to get past fellow American Donald Young 6-2, 6-4.

He will next face third-seeded Spaniard Feliciano Lopez, who defeated Slovakia's Lukas Lacko 7-6 (7/4), 6-1.