Andy Murray beat four-time defending champion Novak Djokovic in straight sets at London's O2 Arena on Sunday to win the ATP World Tour Finals title and also secure year-end top spot in the world rankings.The 29-year-old became the 17th different player to finish the year atop the world in history since 1973 and the first British player to achieve the feat, Xinhua news agency reported.The Scot extended his winning streak to 24 matches dated back to September 18 after a 6-3, 6-4 victory.Murray had spent nine hours and 56 minutes entering the final, including the two longest best-of-three set matches in tournament history, while Djokovic needed three hours less to reach the title match.