Ana Ivanovic of Serbia celebrates a score during the second round match of Porsche Tennis Grand Prix against Nadia Petrova of Russia in Stuttgart, Germany, on April 25, 2013. Ana Ivanovic won 2-0. (Xinhua/Ma Ning)

Ana Ivanovic of Serbia returns the ball during the second round match of Porsche Tennis Grand Prix against Nadia Petrova of Russia in Stuttgart, Germany, on April 25, 2013. Ana Ivanovic won 2-0. (Xinhua/Ma Ning)

Nadia Petrova of Russia returns the ball during the second round match of Porsche Tennis Grand Prix against Ana Ivanovic of Serbia in Stuttgart, Germany, on April 25, 2013. Petrova lost 0-2. (Xinhua/Ma Ning)

Ana Ivanovic of Serbia returns the ball during the second round match of Porsche Tennis Grand Prix against Nadia Petrova of Russia in Stuttgart, Germany, on April 25, 2013. Ana Ivanovic won 2-0. (Xinhua/Ma Ning)

Ana Ivanovic of Serbia returns the ball during the second round match of Porsche Tennis Grand Prix against Nadia Petrova of Russia in Stuttgart, Germany, on April 25, 2013. Ana Ivanovic won 2-0. (Xinhua/Ma Ning)

Ana Ivanovic of Serbia returns the ball during the second round match of Porsche Tennis Grand Prix against Nadia Petrova of Russia in Stuttgart, Germany, on April 25, 2013. Ana Ivanovic won 2-0. (Xinhua/Ma Ning)