Zenits thinking: Why would we sell him for less than we can get otherwise? We dont care where he goes, we just want the highest offer.
Juve's thinking: Why would we offer more for him, he wants to come to us, and they cant move him unless he agrees to go somewhere.
Witsel's thinking: Why wouldnt I demand to go to where I want, they have to sell me this window, or they get nothing for me, and if its 7 mill less, too bad for Zenit, as its better than 0, and Ill just threaten to not move anywhere.
Everton's thinking: We offered the most for him, stand pat, eventually Zenit has to sell to us, as we are much higher than Juve.
Itll all come down to the last week, because up until then everyone holds cards. This may likely be a standoff. Once the last week show up, either Zenit or Witsel have all of the cards. If Zenit can wait out Juve(which they want to do) so that Juve are no longer willing to wait, and buy a CM, and thus no longer have interest in Witsel, Zenit get what they want and sell him to Everton. If it gets to that point and Juve are still interested, Witsel holds the cards, as Zenit then have to choose between a lower fee, or getting 0 for him.