I dont believe Djokovic is injured. He was all over the place, maybe just a way to lure Ferrer to being more aggressive and it worked.
Andy is going to bring up the big serve and the big forehand.
I totally agree, I think he was playing mind games, as the commentators noted the replay of Djokovic "hurting" himself was played roughly 10 times on the big screen in the 15 minutes after it happened. Novak would have known that. Novak is known for exaggerating injuries, 20 minutes after the "injury" he was running about mad hitting winners from everywhere on the court. Andy should prepare for a fully fit and hungry Djokovic.
What is noticeable is that Djokovic has suffered from a serious bouts of nerves in his last two matches against Hewitt and Ferrer, he clearly feels the pressure when he gets into the sort of long rallies Hewitt and Ferrer are notorious for. If Djokovic is off his game on Friday as much as he was for the first two sets, Andy could look to play a similiar style to Hewitt and Ferrer (even know I'm not overly keen on him doing so).
Another big factor is Lendl in Murray's camp. This has been talked about far too much in my opinion, but I do think this could have a big impact on the match, not even so much from Andy's perspective, but of Novak's. Djokovic mentioned in his post-match on court interview earlier that Lendl was in Murray's camp, which proves it is in his mind. Djokovic (and perhaps Federer and Nadal) is wary that Lendl could give Murray the encouragement and incentive he needs to pull off a couple of historic victories and take his first major. He knows that on Friday Andy will bring something(s) to the table that he would have not seen before from a match against Andy. This could play a part in Djokovic's mind and in his tactics, as he will be unsure whether to prepare for the 'usual' Murray or the unknown Murray. The mentality of both players both before and during this match will play a fascinating part in the outcome of the match.
Andy's biggest worry is Djokovic coming out firing, similiar to the way he played in the 3rd set against Ferrer today. As we saw today from Djokovic however is that he came out firing, holding his serves comfortably and taking Ferrer to Deuce in numerous of his service games at the beginning of the match. He didn't miss a ball for the first 6-8 games. Then as the match wore on, and as Ferrer began to impose himself more and more in the match, errors started to gradually creep into Novak's game to the point where in the long rallies Ferrer was outlasting Novak 9 times in every 10. It was only when Ferrer made several mistakes to gift Djokovic the second set tiebreak that Djokovic relaxed and played his best in the 3rd. If Ferrer had taken the 2nd set, it would have been some dogfight. Djokovic will be nervous on Friday (I hope) and hopefully Andy can take advantage.