Jelena Jankovic of Serbia greets the audience during the first round match of Porsche Tennis Grand Prix against Samantha Stosur of Austrilia in Stuttgart, Germany, on April 24, 2013. Jelena Jankovic won 2-0. (Xinhua/Ma Ning)

Jelena Jankovic of Serbia returns the ball during the first round match of Porsche Tennis Grand Prix against Samantha Stosur of Austrilia in Stuttgart, Germany, on April 24, 2013. Jelena Jankovic won 2-0. (Xinhua/Ma Ning)

Petra Kvitova of the Czech Republic returns the ball during the first round match of Porsche Tennis Grand Prix against Annika Beck of Germany in Stuttgart, Germany, on April 24, 2013. Petra Kvitova won 2-1. (Xinhua/Ma Ning)

Petra Kvitova of the Czech Republic returns the ball during the first round match of Porsche Tennis Grand Prix against Annika Beck of Germany in Stuttgart, Germany, on April 24, 2013. Petra Kvitova won 2-1. (Xinhua/Ma Ning)

Petra Kvitova of the Czech Republic returns the ball during the first round match of Porsche Tennis Grand Prix against Annika Beck of Germany in Stuttgart, Germany, on April 24, 2013. Petra Kvitova won 2-1. (Xinhua/Ma Ning)

Petra Kvitova of the Czech Republic serves during the first round match of Porsche Tennis Grand Prix against Annika Beck of Germany in Stuttgart, Germany, on April 24, 2013. Petra Kvitova won 2-1. (Xinhua/Ma Ning)

Annika Beck of Germany returns the ball during the first round match of Porsche Tennis Grand Prix against Petra Kvitova of the Czech Republic in Stuttgart, Germany, on April 24, 2013. Beck lost the match 1-2.(Xinhua/Ma Ning)

Samantha Stosur of Austrilia returns the ball during the first round match of Porsche Tennis Grand Prix against Jelena Jankovic of Serbia in Stuttgart, Germany, on April 24, 2013. Samantha Stosur lose the match 0-2. (Xinhua/Ma Ning)

Petra Kvitova of the Czech Republic greets the audience after the first round match of Porsche Tennis Grand Prix against Annika Beck of Germany in Stuttgart, Germany, on April 24, 2013. Petra Kvitova won 2-1. (Xinhua/Ma Ning)

Samantha Stosur of Austrilia returns the ball during the first round match of Porsche Tennis Grand Prix against Jelena Jankovic of Serbia in Stuttgart, Germany, on April 24, 2013. Samantha Stosur lose the match 0-2. (Xinhua/Ma Ning)

Petra Kvitova of the Czech Republic returns the ball during the first round match of Porsche Tennis Grand Prix against Annika Beck of Germany in Stuttgart, Germany on April 24, 2013. Petra Kvitova won 2-1. (Xinhua/Ma Ning)

Samantha Stosur of Austrilia returns the ball during the first round match of Porsche Tennis Grand Prix against Jelena Jankovic of Serbia in Stuttgart, Germany, on April 24, 2013. Samantha Stosur lose the match 0-2. (Xinhua/Ma Ning)

Annika Beck of Germany returns the ball during the first round match of Porsche Tennis Grand Prix against Petra Kvitova of the Czech Republic in Stuttgart, Germany, on April 24, 2013. Beck lost the match 1-2. (Xinhua/Ma Ning)

Jelena Jankovic of Serbia returns the ball during the first round match of Porsche Tennis Grand Prix against Samantha Stosur of Austrilia in Stuttgart, Germany, on April 24, 2013. Jelena Jankovic won 2-0. (Xinhua/Ma Ning)

Jelena Jankovic of Serbia returns the ball during the first round match of Porsche Tennis Grand Prix against Samantha Stosur of Austrilia in Stuttgart, Germany, on April 24, 2013. Jelena Jankovic won 2-0. (Xinhua/Ma Ning)