Rafael Nadal of Spain talks to Roger Federer of Switzerland after their Singles Semifinal on day 7 of the Barclays ATP World Tour Finals at the O2 Arena in London, Britain, Nov. 10, 2013. Rafael Nadal won 2-0. (Xinhua/Wang Lili)

Novak Djokovic of Serbia celebrates after winning the Singles Semifinal against Stanislas Wawrinka of Switzerland on day 7 of the Barclays ATP World Tour Finals at the O2 Arena in London, Britain, Nov. 10, 2013. Novak Djokovic won 2-0. (Xinhua/Wang Lili)

Novak Djokovic of Serbia returns the ball during the Singles Semifinal against Stanislas Wawrinka of Switzerland on day 7 of the Barclays ATP World Tour Finals at the O2 Arena in London, Britain, Nov. 10, 2013. Novak Djokovic won 2-0. (Xinhua/Wang Lili)

Novak Djokovic of Serbia celebrates after winning the Singles Semifinal against Stanislas Wawrinka of Switzerland on day 7 of the Barclays ATP World Tour Finals at the O2 Arena in London, Britain, Nov. 10, 2013. Novak Djokovic won 2-0. (Xinhua/Wang Lili)

Roger Federer of Switzerland reacts during the Singles Semifinal against Rafael Nadal of Spain on day 7 of the Barclays ATP World Tour Finals at the O2 Arena in London, Britain, Nov. 10, 2013. Roger Federer lost 0-2. (Xinhua/Wang Lili)

Rafael Nadal of Spain celebrates after winning the Singles Semifinal against Roger Federer of Switzerland on day 7 of the Barclays ATP World Tour Finals at the O2 Arena in London, Britain, Nov. 10, 2013. Rafael Nadal won 2-0. (Xinhua/Wang Lili)

Rafael Nadal of Spain returns the ball during the Singles Semifinal against Roger Federer of Switzerland on day 7 of the Barclays ATP World Tour Finals at the O2 Arena in London, Britain, Nov. 10, 2013. Rafael Nadal won 2-0. (Xinhua/Wang Lili)

Rafael Nadal of Spain returns the ball during the Singles Semifinal against Roger Federer of Switzerland on day 7 of the Barclays ATP World Tour Finals at the O2 Arena in London, Britain, Nov. 10, 2013. Rafael Nadal won 2-0. (Xinhua/Wang Lili)

Rafael Nadal of Spain returns the ball during the Singles Semifinal against Roger Federer of Switzerland on day 7 of the Barclays ATP World Tour Finals at the O2 Arena in London, Britain, Nov. 10, 2013. Rafael Nadal won 2-0. (Xinhua/Wang Lili)

Novak Djokovic of Serbia returns the ball during the Singles Semifinal against Stanislas Wawrinka of Switzerland on day 7 of the Barclays ATP World Tour Finals at the O2 Arena in London, Britain, Nov. 10, 2013. Novak Djokovic won 2-0. (Xinhua/Wang Lili)

Novak Djokovic of Serbia returns the ball during the Singles Semifinal against Stanislas Wawrinka of Switzerland on day 7 of the Barclays ATP World Tour Finals at the O2 Arena in London, Britain, Nov. 10, 2013. Novak Djokovic won 2-0. (Xinhua/Wang Lili)

Novak Djokovic of Serbia returns the ball during the Singles Semifinal against Stanislas Wawrinka of Switzerland on day 7 of the Barclays ATP World Tour Finals at the O2 Arena in London, Britain, Nov. 10, 2013. Novak Djokovic won 2-0. (Xinhua/Wang Lili)