STUTTGART, GERMANY - JUNE 11: Roger Federer of Switzerland returns against Dominic Thiem of Austria during the semifinals on day 8 of Mercedes Cup 2016 on June 11, 2016 in Stuttgart, Germany. (Photo by Daniel Kopatsch/Bongarts/Getty Images)
STUTTGART, GERMANY - JUNE 11: Roger Federer of Switzerland returns against Dominic Thiem of Austria during the semifinals on day 8 of Mercedes Cup 2016 on June 11, 2016 in Stuttgart, Germany. (Photo by Daniel Kopatsch/Bongarts/Getty Images)