Well, the story I heard was that Djokovic's and Seles' first coach was a woman named Jelena Gencic. She was either a coach or the woman who first identified them as "extraordinary" or something of that nature.
You r right. They both worked with Jelena Gencic as their first coach, but being the one that had the same opportunity 15 years ago, I can just say that I can see the traces of her influence both in Monika or Nole but not as decisive. First of all, two-handed backhand. I don't know anybody who worked with Gencic that didn't adopted this style. Further, she insists on top-spins, a lot of it in every hit. Monika never adopted this shot. Nole did, but I think it hasn't much to do with Jelena. Kick serve is another similarity, but as you may recall, Monika wasn't very good in it. Volley game, or lack of it, is maybe the best example, but as we all saw, Nole started to improve. There is one more thing. Nole knows how to slide, but Monika was the best in it in her time. She was one of the best on clay (Jelena conducts her trainings on clay predominantly).