CHARLESTON — Defending champion Andrea Petkovic booked her quarterfinal berth at the WTA tournament in Charleston Thursday, improving her perfect record in the event with a 6-4, 6-4 win over Madison Brengle. 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 Eugenie Bouchard in straight sets 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. Davis next faces seventh-seeded American Madison Keys. Keys, an Australian Open semifinalist, defeated Romanian Andreea Mitu 6-2, 6-0. 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. Sock surprises Bautista In Houston, unseeded American Jack Sock toppled second-seeded Spaniard Roberto Bautista Agut 6-4, 6-4 Thursday to reach the quarter-finals of the ATP clay court tournament in Houston. Sock, ranked 46th in the world, notched his second victory over the 15th-ranked Spaniard in less than a month. Sock 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. Also taking the court Thursday were two former tournament winners, including defending champ Fernando Verdasco who beat South Korean teen Chung Hyeon in straight sets, 7-6 (7-5), 6-2. But John Isner, who won two years ago, failed to advance, losing 6-2, 7-6 (7-2) to Russian Teymuraz Gabashvili. Verdasco blasted two aces, had just one double fault and broke the 18-year-old Chung twice in the 92-minute match. He moves on to play Gabashvili in the quarters. Chung was the sixth teenager to qualify for an ATP tour event in 2015. The world No. 118 qualified for his first Masters level main draw in Miami, winning his first-round match before falling to his first top-10 opponent, Czech Tomas Berdych. Radwanska advances In Poland, top-seeded Agnieszka Radwanska of Poland beat Belgian An-Sophie Mestach 6-3, 6-1 at the Katowice Open Thursday. Radwanska will face Klara Koukalova of the Czech Republic in the quarterfinals Friday. Earlier, Elizaveta Kulichkova of Russia defeated fifth-seeded Kaia Kanepi 6-1, 6-2, and is now set to face Camila Giorgi. Eighth-seeded Anna Schmiedlova of Slovakia need three sets to see off Katerina Siniakova of the Czech Republic, 6-1, 3-6, 6-3, and plays her next match against Alize Cornet. Belgium's Kirsten Flipkens, the seventh-seed, also took three sets to rout Monica Niculescu of Romania, 6-7 (6), 6-1, 6-4. — Agencies
