I think I have to agree with Bobby Jr. here. I seriously doubt Djokovic would be wearing adidas shoes without a deal of some sort with them. Why would he want to basically give away advertising for adidas for free? Seeing as he wears adidas shoes on court, in the gym, and for casual use, it seems as if he must have a deal of some sort with them. Added to that is the fact that his brother Marko has exactly the same thing going; Tacchini clothing and adidas shoes. Also his team all wear adidas training shoes, as well as the fact that he was wearing custom miAdidas Barricades in his national colours in Australia, which was another great advertisement for adidas. Obviously there isn't really anything that supports what I'm saying, merely an observation that it wouldn't make sense for him to wear adidas without a shoe deal with them of some sort.
It would be interesting to see what kind of shoe deal (if any) the guys who wear brands that don't make performance shoes have, e.g. Lacoste, Tacchini. Someone on the board (I think it was Fee) said guys like Gasquet and Benneteau, who wear lacoste apparel and Asics shoes, just get the shoes on a tournament by tournament basis. They get the shoes (and I assume some payment for endorsing them) at the beginning of the tournament because Asics are trying to build their reputation, and are more willing to supply those who don't have a shoe sponsor. So they may not have a permanent deal with them.
But then someone else like Llodra or Raonic who also wear Lacoste but then Nike shoes may have some sort of deal with Nike, as they seem to wear exclusively Nike shoes. It's the same for a lot of the doubles players - guys like Nestor, Lukas Dlouhy, Zimonjic (I think) all seem to wear Nike shoes, but different apparel sponsors. Tommy Haas also has been wearing Nike shoes for ages, (he was previously sponsored by them) even while he was with K Swiss and after he switched to that other sponsor he was with. And he was wearing shoes out of production, like older models that are harder to get hold of. Or maybe that were on sale if he was buying them himself. But then I recall seeing Benneteau wearing Nike shoes, so it could be that the Nike shoe guys are just buying them.
Then there's Wawrinka, who used to be with adidas, changed to lacoste, but still wears Adidas shoes in exactly the same way as Djokovic. Maybe due to former ties with adidas, they are willing to supply Stan and Nole with shoes for free as they are bigger name players who will generally go further in tournaments and therefore end up promoting the shoes more.