It's really hard to call the Fed vs. Djokovic match. As I said before, if FedError turns up then Djokovic will take it in three. However, if FedEx turns up then I think that the old man will win in either four or five (probably five). Federer would rather have a leg off than lose to the Serb on Centre Court (his favourite court), so I think that he's going to put every ounce of effort into beating Djokovic. Either way, I think both men should treat this semi as if it were a final, as the winner of this match is heavy favourite to take the whole thing.
I really do think that Murray will come through against Tsonga, probably in four sets. Murray has a strong record against Tsonga and should still be able to deal with the Frenchman even if he does catch fire with his serve.
If Djokovic wins his semi-final then it will be very difficult to see past him winning the whole thing. Murray is more likely to challenge him than Tsonga is, but even then I fail to see how either player can truly hurt him. If Federer wins, I think that he would rather play Murray than Tsonga. While Murray is the better player, there is always the chance that Tsonga could just fire bullets at Federer for the whole match (a la Wimbledon 2011) and that old man just won't be able to deal with it. Murray would be mentally fragile (due to pressure of being the hope of the nation) and his defensive mode of play is likely to be exploited by an in-form Federer.