Li Na of China returns the ball during the quarter final match of Porsche Tennis Grand Prix against Petra Kvitova of the Czech Republic in Stuttgart, Germany, on April 26, 2013. Li Na won 2-0. (Xinhua/Ma Ning)

Li Na of China returns the ball during the quarter final match of Porsche Tennis Grand Prix against Petra Kvitova of the Czech Republic in Stuttgart, Germany, on April 26, 2013. Li Na won 2-0. (Xinhua/Ma Ning)

Li Na of China serves during the quarter final match of Porsche Tennis Grand Prix against Petra Kvitova of the Czech Republic in Stuttgart, Germany, on April 26, 2013. Li Na won 2-0. (Xinhua/Ma Ning)

Li Na of China returns the ball during the quarter final match of Porsche Tennis Grand Prix against Petra Kvitova of the Czech Republic in Stuttgart, Germany, on April 26, 2013. Li Na won 2-0. (Xinhua/Ma Ning)

Li Na of China serves during the quarter final match of Porsche Tennis Grand Prix against Petra Kvitova of the Czech Republic in Stuttgart, Germany, on April 26, 2013. Li Na won 2-0. (Xinhua/Ma Ning)

Li Na (R) of China is interviewed after winning the quarter final match of Porsche Tennis Grand Prix against Petra Kvitova of the Czech Republic in Stuttgart, Germany, on April 26, 2013. Li Na won 2-0. (Xinhua/Ma Ning)

Li Na of China greets the audience after winning the quarter final match of Porsche Tennis Grand Prix against Petra Kvitova of the Czech Republic in Stuttgart, Germany, on April 26, 2013. Li Na won 2-0. (Xinhua/Ma Ning)

Li Na of China reacts after winning the quarter final match of Porsche Tennis Grand Prix against Petra Kvitova of the Czech Republic in Stuttgart, Germany, on April 26, 2013. Li Na won 2-0. (Xinhua/Ma Ning)

Petra Kvitova of the Czech Republic returns the ball during the quarter final match of Porsche Tennis Grand Prix against Li Na of China in Stuttgart, Germany, on April 26, 2013. Li Na won 2-0. (Xinhua/Ma Ning)