The German won seven career WTA singles titles and five doubles titles, qualifying for the WTA Finals with Karolina Pliskova in 2016 and finishing runner-up at the 2014 French Open mixed doubles event alongside Nenad Zimonjic.
"You have given me so many different types of emotions throughout our journey and I am very thankful for everything you have shown and taught me. I learnt how to deal with the toughest losses but also enjoy the most amazing wins of my career, to fight back many times when I was struggling with you and through it all we never lost sight of our dreams.
"I always knew how I would feel when it is time to say goodbye to you, and that moment has arrived. I am ready to close the tennis chapter of my life and open a new one, which I am really excited about.
"Thank you for everything you have given me. You will stay forever in my heart."