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STUTTGART, GERMANY - APRIL 23: (L-R) Laura Siegmund, Carina Witthoeft, Angelique Kerber, Julia Goerges and Barbara Rittner of Germany celebrate victory during the FedCup World Group Play-Off match between Germany and Ukraine at Porsche Arena on April 23, 2017 in Stuttgart, Germany. (Photo by Simon Hofmann/Bongarts/Getty Images)

STUTTGART, GERMANY - APRIL 23:  (L-R) Laura Siegmund, Carina Witthoeft, Angelique Kerber, Julia Goerges and Barbara Rittner of Germany celebrate victory during the FedCup World Group Play-Off match between Germany and Ukraine at Porsche Arena on April 23, 2017 in Stuttgart, Germany.  (Photo by Simon Hofmann/Bongarts/Getty Images)

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The German Fed Cup team ahead of the draw for the play-offs between Germany and the Ukraine in Stuttgart, Germany, 21 April 2017, l-r: Barbara Rittner, Carina Witthoeft, Julia Goerges, Angelique Kerber and Laura Siegemund. Photo: Daniel Maurer/dpa | usage worldwide (Photo by Daniel Maurer/picture alliance via Getty Images)

The German Fed Cup team ahead of the draw for the play-offs between Germany and the Ukraine in Stuttgart, Germany, 21 April 2017, l-r: Barbara Rittner, Carina Witthoeft, Julia Goerges, Angelique Kerber and Laura Siegemund. Photo: Daniel Maurer/dpa | usage worldwide   (Photo by Daniel Maurer/picture alliance via Getty Images)

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BRISBANE, AUSTRALIA - APRIL 17: The German Fed Cup team (L-R) Coach Barbara Rittner, Anna Petkovic, Anna-Lena Groenefeld, Julia Goerges and Angelique Kerber pose for a photo during the Fed Cup official dinner ahead of the Semi Final tie between Australia and Germany at the Stamford Plaza on April 17, 2014 in Brisbane, Australia. (Photo by David Hardenberg/Getty Images)

BRISBANE, AUSTRALIA - APRIL 17: The German Fed Cup team (L-R)  Coach Barbara Rittner, Anna Petkovic, Anna-Lena Groenefeld, Julia Goerges and Angelique Kerber pose for a photo during the Fed Cup official dinner ahead of the Semi Final tie between Australia and Germany at the Stamford Plaza on April 17, 2014 in Brisbane, Australia. (Photo by David Hardenberg/Getty Images)

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The German team with Julia Goerges (l-r), Angelique Kerber, Laura Siegemund, Carina Witthoeft and team head Barbara Rittner celebrates remaining in the FedCup world group after the Federation Cup world group relegation play-off round tennis match between Germany and Ukraine in Stuttgart, Germany, 23 April 2017. Photo: Daniel Maurer/dpa | usage worldwide (Photo by Daniel Maurer/picture alliance via Getty Images)

The German team with Julia Goerges (l-r), Angelique Kerber, Laura Siegemund, Carina Witthoeft and team head Barbara Rittner celebrates remaining in the FedCup world group after the Federation Cup world group relegation play-off round tennis match between Germany and Ukraine in Stuttgart, Germany, 23 April 2017. Photo: Daniel Maurer/dpa | usage worldwide   (Photo by Daniel Maurer/picture alliance via Getty Images)