I think there'are two main reasons of his current form:
1) Physical improvement
2) Ability to keep the highest concentration through a very long period of time
Actually from the beginning of D"Joke's career I considered him as a player with great mentality, this type of mentality which allows you to win usually tight matches. Recently, he wasn't forced to play a tight match even once! I mean in the winning period, starting with the Davis Cup final. During the 20-match winning streak (or 23 with Hopman Cup), he played deciding set only 4 times: three times lost only two games (Berdych, Federer, Nadal), once four games (F.Lopez), but even in that match, he wasn't close to lose, led early with a break in the 3rd set.
He has been in this beautiful period characterized by enormous will to play tennis. He is dangerous at 0-40 in almost every game, whether he serves or receives doesn't really matter. Federer and Nadal experienced the same thing in their best periods, obviously this state of mind can't last forever, but the tennis history teaches us, it can last over 40 matches, so D'Joke theoretically is the halfway at the moment