Novak Djokovic has now won the ATP World Tour Finals twice. The first time was in 2008 when he beat Nikolai Davydenko in straight sets. He has now won for the second time in 2012, beating Roger Federer (again in straight sets) who had previously won the championship several times and was the reigning champion. 2012 is the 6th time Djokovic has qualified for the ATP.
This victory has returned Djokovic to the world no. 1 slot. He won both Wimbledon and the US Open in 2011, and has also won the Australian Open three times. He has also won several other titles and, at only 25, many well win several more.
2012 has been a difficult year for Djokovic, whose father is suffering from a serious illness. After winning the ATP, Novak has said that his father inspired him to this victory, and has dedicated his win to him.