Boris Becker, top centre, looks on as Novak Djokovic serves against Roger Federer during their semi-final match at the Dubai Duty Free Tennis Championships on February 28, 2014. Marwan Naamani / AFP
Novak Djokovic of Serbia reacts during his semi-final against Roger Federer at the Dubai Duty Free Tennis Championships on February 28, 2014. Ahmed Jadallah / Reuters
Novak Djokovic returns a shot to Roger Federer during their semi-final match at the Dubai Duty Free Tennis Championships on February 28, 2014. Kamran Jebreili / AP Photo
Roger Federer plays a backhand against Novak Djokovic during the semi-finals of the Dubai Duty Free Tennis Championships at Dubai Tennis Stadium on February 28, 2014. Sarah Dea / The National
Novak Djokovic, left, and Roger Federer pose before their semi-final at the Dubai Duty Free Tennis Championships at Dubai Tennis Stadium on February 28, 2014. Sarah Dea / The National
Novak Djokovic returns the ball to Roger Federer during their semi-final at the Dubai Duty Free Tennis Championships on February 28, 2014. Marwan Naamani / AFP
Roger Federer returns the ball to Novak Djokovic during their semi-final at the Dubai Duty Free Tennis Championships on February 28, 2014. Karim Sahib / AFP