Top seed John Isner advanced to the semi-finals of the Fayez Sarofim & Co. U.S. Men's Clay Court Championship in Houston with a straight-sets win over Hyeon Chung.

Isner sent down 14 aces and saved all seven break points he faced to set up a last-four clash with Jack Sock after the 7-6 (7/5) 6-4 win.

"That one game in the first set, I was down four break points," he said. "Serving at 4-3 (in the second set), I was down three break points and I had to hit a stab volley on one of them. That was huge. He played a good return game."

Defending champion Sock extended his streak at Houston to 14 straight sets won as he beat Cyprus' Marcos Baghdatis 6-4 6-4.

"I've now won a few matches here and I feel great out there on that court," said Sock. "It's definitely one of my favourite courts to play on throughout the year, always feel comfortable out there, so confidence is a little higher."

Juan Monaco moved into the semi-finals with a 6-4 6-4 win over Sam Querrey and he will face third seed Feliciano Lopez, who saw off Tim Smyczek 6-2 6-3.