STUTTGART, GERMANY - JUNE 10: Florian Mayer of Germany returns against Roger Federer of Switzerland during the quarterfinals on day 7 of Mercedes Cup 2016 on June 10, 2016 in Stuttgart, Germany. (Photo by Daniel Kopatsch/Bongarts/Getty Images)
STUTTGART, GERMANY - JUNE 10: Florian Mayer of Germany returns against Roger Federer of Switzerland during the quarterfinals on day 7 of Mercedes Cup 2016 on June 10, 2016 in Stuttgart, Germany. (Photo by Daniel Kopatsch/Bongarts/Getty Images)