Novak Djokovic of Serbia greets the spectators after winning the Singles Group B match against Roger Federer of Switzerland on day 2 of the Barclays ATP World Tour Finals at the O2 Arena in London, Britain, Nov. 5, 2013. Novak Djokovic won 2-1. (Xinhua/Wang Lili) 
Novak Djokovic of Serbia celebrates after winning the Singles Group B match against Roger Federer of Switzerland on day 2 of the Barclays ATP World Tour Finals at the O2 Arena in London, Britain, Nov. 5, 2013. Novak Djokovic won 2-1. (Xinhua/Wang Lili) 
Novak Djokovic of Serbia returns the ball during the Singles Group B match against Roger Federer of Switzerland on day 2 of the Barclays ATP World Tour Finals at the O2 Arena in London, Britain, Nov. 5, 2013. Novak Djokovic won 2-1. (Xinhua/Wang Lili) 
Novak Djokovic of Serbia serves during the Singles Group B match against Roger Federer of Switzerland on day 2 of the Barclays ATP World Tour Finals at the O2 Arena in London, Britain, Nov. 5, 2013. Novak Djokovic won 2-1. (Xinhua/Wang Lili) 
Novak Djokovic of Serbia returns the ball during the Singles Group B match against Roger Federer of Switzerland on day 2 of the Barclays ATP World Tour Finals at the O2 Arena in London, Britain, Nov. 5, 2013. Novak Djokovic won 2-1. (Xinhua/Wang Lili) 
Novak Djokovic of Serbia returns the ball during the Singles Group B match against Roger Federer of Switzerland on day 2 of the Barclays ATP World Tour Finals at the O2 Arena in London, Britain, Nov. 5, 2013. Novak Djokovic won 2-1. (Xinhua/Yin Gang) 
Novak Djokovic of Serbia returns the ball during the Singles Group B match against Roger Federer of Switzerland on day 2 of the Barclays ATP World Tour Finals at the O2 Arena in London, Britain, Nov. 5, 2013. Novak Djokovic won 2-1. (Xinhua/Yin Gang) 
Novak Djokovic of Serbia celebrates during the Singles Group B match against Roger Federer of Switzerland on day 2 of the Barclays ATP World Tour Finals at the O2 Arena in London, Britain, Nov. 5, 2013. Novak Djokovic won 2-1. (Xinhua/Yin Gang) 
Novak Djokovic of Serbia gestures during the Singles Group B match against Roger Federer of Switzerland on day 2 of the Barclays ATP World Tour Finals at the O2 Arena in London, Britain, Nov. 5, 2013. Novak Djokovic won 2-1. (Xinhua/Wang Lili) 
Novak Djokovic of Serbia walks to the court ahead of the Singles Group B match against Roger Federer of Switzerland on day 2 of the Barclays ATP World Tour Finals at the O2 Arena in London, Britain, Nov. 5, 2013. Novak Djokovic won 2-1. (Xinhua/Wang Lili) 
Roger Federer of Switzerland returns the ball during the Singles Group B match against Novak Djokovic of Serbia on day 2 of the Barclays ATP World Tour Finals at the O2 Arena in London, Britain, Nov. 5, 2013. Federer lost 1-2. (Xinhua/Wang Lili) 
Roger Federer of Switzerland reacts during the Singles Group B match against Novak Djokovic of Serbia on day 2 of the Barclays ATP World Tour Finals at the O2 Arena in London, Britain, Nov. 5, 2013. Federer lost 1-2. (Xinhua/Wang Lili) 
Roger Federer of Switzerland walks to the court ahead of the Singles Group B match against Novak Djokovic of Serbia on day 2 of the Barclays ATP World Tour Finals at the O2 Arena in London, Britain, Nov. 5, 2013. Federer lost 1-2. (Xinhua/Wang Lili) 
Roger Federer of Switzerland returns the ball during the Singles Group B match against Novak Djokovic of Serbia on day 2 of the Barclays ATP World Tour Finals at the O2 Arena in London, Britain, Nov. 5, 2013. Federer lost 1-2. (Xinhua/Wang Lili) 
Roger Federer of Switzerland returns the ball during the Singles Group B match against Novak Djokovic of Serbia on day 2 of the Barclays ATP World Tour Finals at the O2 Arena in London, Britain, Nov. 5, 2013. Federer lost 1-2. (Xinhua/Yin Gang) |
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