The Spaniard has won nine titles since returning from a long-term knee problem, including hard-court titles at Indian Wells, the Rogers Cup and in Cincinnati.
His victories have propelled him to second in the rankings, 2,120 points behind the Serbian, with the pair possibly renewing their rivalry in the final at Flushing Meadows.
When asked about his chances of finishing the year as number one, Djokovic told Sky Sports News: "It's really tough to say.
"Nowadays, players at the top play for the whole year, which is what makes it challenging to finish on top. Rafa coming back and playing so well, that's what makes him favourite.
"But I still have [the number one ranking] and hopefully I can keep it, because Andy [Murray] has been playing well, and so have other players like Tomas Berdych. This is what you work for."