" Roger Federer of Switzerland returns the ball during the match against Novak Djokovic of Serbia at the ATP World Tour Finals at the O2 Arena in London vans old skool grise , Nov. 17, 2015. Federer won 2-0. (XinhuaHan Yan) Defending Champion Novak Djokovic of Serbia returns the ball during the match against Roger Federer of Switzerland at the ATP World Tour Finals at the O2 Arena in London, Nov. 17, 2015. Novak Djokovic lost 0-2. (XinhuaHan Yan) Roger Federer of Switzerland returns the ball during the match against Novak Djokovic of Serbia at the ATP World Tour Finals at the O2 Arena in London vans old skool blanche , Nov. 17, 2015. Federer won 2-0. (XinhuaHan Yan) Roger Federer of Switzerland returns the ball during the match against Novak Djokovic of Serbia at the ATP World Tour Finals at the O2 Arena in London, Nov. 17, 2015. Federer won 2-0. (XinhuaHan Yan) Defending Champion Novak Djokovic of Serbia returns the ball during the match against Roger Federer of Switzerland at the ATP World Tour Finals at the O2 Arena in London vans old skool blanche cuir , Nov. 17, 2015. Novak Djokovic lost 0-2. (XinhuaHan Yan) Defending Champion Novak Djokovic of Serbia reacts during the match against Roger Federer of Switzerland at the ATP World Tour Finals at the O2 Arena in London, Nov. 17, 2015. Novak Djokovic lost 0-2. (XinhuaHan Yan) Roger Federer (L) of Switzerland and Novak Djokovic of Serbia are seen during the match against Novak Djokovic of Serbia at the ATP World Tour Finals at the O2 Arena in London