HOUSTON: Defending champion Fernando Verdasco beat 18-year-old Hyeon Chung 7-6 (2), 6-2 on Thursday in the second round of the US 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.

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.

Unseeded American Jack Sock toppled second-seeded Spaniard Roberto Bautista Agut 6-4, 6-4 to reach the quarterfinals.

Sock, ranked 46th in the world, notched his second victory over the 15th-ranked Spaniard in less than a month.

He also beat Bautista Agut in the round of 32 in the Indian Wells Masters in March.

Sock, who lifted the doubles title with Vasek Pospisil in Indian Wells and reached the final with the Canadian at Miami last weekend, saved both break points he faced in the one hour, 21 minute clash.

He broke the Spaniard once in each set to secure the win and next faces fifth-seeded Colombian Santiago Giraldo who toppled unseeded Serbian Janko Tipsarevic 7-6 (7/2), 6-3.

Tipsarevic, once ranked as high as eighth in the world, is on the comeback trail after missing 17 months in the wake of multiple surgeries on his left foot.

On Tuesday the Serb saved two match points en route to a three-set win over Guilherme Clezar.

John Isner, who won two years ago, failed to advance, losing 6-2, 7-6 (7/2) to Russian Teymuraz Gabashvili.

Petkovic in quarters

In Charleston, defending champion Andrea Petkovic booked her quarterfinal berth at the WTA tournament in Charleston on Thursday, improving her perfect record in the event with a 6-4, 6-4 win over Madison Brengle.

The third-seeded German improved to 10-0 on the green clay of Charleston, avenging her first-round loss to Brengle at the Australian Open.

Petkovic, ranked 11th in the world, is the highest-seeded player left in the draw after the second-round exit of top-seeded Canadian Eugenie Bouchard and the withdrawal of second-seeded Ekaterina Makarova with a stomach ailment.

Petkovic will next face qualifier Danka Kovinic of Montenegro, who reached the quarters when former champion and sixth seed Jelena Jankovic withdrew from their scheduled third-round match with a right foot injury.

Lauren Davis, who stunned Bouchard in straight sets on Wednesday, moved into the quarterfinals with an abbreviated victory over ailing Mona Barthel.

Davis was leading the 15th-seeded German 6-4, 3-0 when Barthel retired because of dizziness.

Although she hasn’t spent all that much time on court, the 66th ranked Davis said she was finding her feet on Charleston’s clay courts.

Davis next faces seventh-seeded American Madison Keys. Keys, an Australian Open semifinalist, defeated Romanian Andreea Mitu 6-2, 6-0.

Davis and Keys are familiar foes. With their career head-to-head record tied at two victories apiece.

Fourth-seeded Italian Sara Errani reached the quarterfinals with a 6-2, 5-7, 6-2 victory over Spain’s Sara Sorribes Tormo.

Errani will face Czech Lucie Hradecka, who toppled French eighth seed Caroline Garcia 5-7, 7-5, 6-4.

Fifth-seeded German Angelique Kerber eased past Spain’s Lara Arruabarrena 6-3, 6-0 to line up a quarterfinal meeting with Romania Irina-Camelia Begu. Begu advanced by walkover thanks to Makarova’s decision to pull out.