(CNN) Andy Murray this week ended Novak Djokovic's two-year reign atop the rankings in men's tennis but the Scot's stay at No. 1 could be a short one -- and he knows it. "I might as well try and enjoy it because I could lose it at the tour finals and never be there again," Murray told reporters, referring to the year-end championships, which start Sunday in London. Indeed, Djokovic can regain the top position on Nov. 21 and finish the season as No. 1 for the third straight time with a strong showing at the World Tour Finals. The 29-year-old has certainly fared well at the O2 Arena, winning a record four consecutive titles and if he makes it five in a row while winning at least two of his round-robin matches, Djokovic is guaranteed to seize back the No. 1 ranking from Murray.World Tour Finals start Sunday in LondonDjokovic can return to No. 1Murray has never reached the final at the eventRoger Federer and Rafael Nadal absent
_ surely won't be taking his group rivals lightly: Stan Wawrinka has beaten Murray in three of their past four meetings, with Cilic and Kei Nishikori getting the better of Murray in August and September, respectively. Murray surely won't be taking his group rivals lightly: Stan Wawrinka has beaten Murray in three of their past four meetings, with Cilic and Kei Nishikori getting the better of _ in August and September, respectively. Some, too, would argue that if he hadn't sustained an injury he would have ousted _ after holding a 2-1 lead in sets.