Agnieszka Radwanska of Poland looks dejected in her match against Laura Siegemund of Germany during Day 6 of the Porsche Tennis Grand Prix at Porsche-Arena on April 23, 2016 in Stuttgart, Germany.

Laura Siegemund of Germany shakes hands with Agnieszka Radwanska of Poland after their semi final match during Day 6 of the Porsche Tennis Grand Prix at Porsche-Arena on April 23, 2016 in Stuttgart, Germany.

Laura Siegemund of Germany celebrates match point in her semi final match against Agnieszka Radwanska of Poland during Day 6 of the Porsche Tennis Grand Prix at Porsche-Arena on April 23, 2016 in Stuttgart, Germany.

Agnieszka Radwanska of Poland serves in her semi final match against Laura Siegemund of Germany during Day 6 of the Porsche Tennis Grand Prix at Porsche-Arena on April 23, 2016 in Stuttgart, Germany.

Laura Siegemund of Germany plays a backhand in her semi final match against Agnieszka Radwanska of Poland during Day 6 of the Porsche Tennis Grand Prix at Porsche-Arena on April 23, 2016 in Stuttgart, Germany.

Laura Siegemund of Germany celebrates a point in her semi final match against Agnieszka Radwanska of Poland during Day 6 of the Porsche Tennis Grand Prix at Porsche-Arena on April 23, 2016 in Stuttgart, Germany.

Angelique Kerber of Germany celebrates match point in her semi final match against Petra Kvitova of Czech Republic during Day 6 of the Porsche Tennis Grand Prix at Porsche-Arena on April 23, 2016 in Stuttgart, Germany.

Angelique Kerber of Germany celebrates a point in her semi final match against Petra Kvitova of Czech Republic during Day 6 of the Porsche Tennis Grand Prix at Porsche-Arena on April 23, 2016 in Stuttgart, Germany.

Petra Kvitova of Czech Republic plays a forehand in her semi final match against Angelique Kerber of Germany during Day 6 of the Porsche Tennis Grand Prix at Porsche-Arena on April 23, 2016 in Stuttgart, Germany.