Petkovic , bothered by a bad knee , dropped her first set Thursday .
Petkovic , bothered by a bad knee , rallied to beat China 's Zheng Jie 3 - 6 , 6 - 3 , 6 - 3 in the second round .
Petkovic , bothered by a bad knee , dropped her first set Thursday but rallied to beat China 's Zheng Jie 3 - 6 , 6 - 3 , 6 - 3 in the second round .