Why are people saying that Djokovic has 'sealed' the #1 ATP ranking?

With his win over Tsonga today, I heard the announcer say that he's sealed his #1 ATP ranking. While I understand that he was only 70 points behind Nadal, Nadal hasn't played against Murray. If Nadal wins, won't he remain #1 (because he gets the same number of points as Djokovic)?