Defending champion Fernando Verdasco beat 18-year-old Hyeon Chung 7-6 (2), 6-2 on Thursday in the second round of the U.S. Men's Clay Court Championship.

The sixth-seeded Verdasco, from Spain, survived stiff early resistance from Chung, then broke the South Korean player in the fifth game of the second set and coasted to the victory.

Verdasco — at 32, 14 years Chung's senior — couldn't solve his opponent's serve in the first set, but that first break in the second set seemed to demoralize the teenager.

"After the first break of the match maybe he went down mentally," Verdasco said. "But he plays really good to be 18. He moves well and has strong legs. He has a good forehand and a good backhand and serves all the time at 120-something. He's pretty complete. With a little more experience, he's going to go high in the rankings."

Chung was the first South Korean player to win a main-draw match in the tournament since Hyung-Taik Lee reached the final in 2001.

American Jack Sock upset second-seeded Roberto Bautista Agut of Spain 6-4, 6-4.

The 22-year-old Sock is playing his third tournament of the year after having surgery to repair a torn muscle in his pelvis in December.

"Tricky conditions today," Sock said. "Windy, sun in and out of the clouds. A lot going on. But I was fortunate. I played well and got through. But I was able to dictate play with my forehand and the clay helps my kick serves. They get up out of his strike zone and he likes everything in the pocket."

In other second-round matches, Russia's Teymuraz Gabashvili beat fourth-seeded John Isner of the United States 6-2, 7-6 (2), and fifth-seeded Santiago Giraldo of Colombia topped Janko Tipsarevic of Serbia 7-6 (2), 6-3.

In the quarterfinals, Verdasco will face Gabashvili, and Sock will play Giraldo.