Blake looks slightly uncomfortable on the clay; as though he doesn't really believe he can win matches on this surface. He's still got his weapons - the serve, huge forehand and speed around the court but there's uncertainty about how to construct the points and when to go for a winner. From Blake's perspective the most sensible approach could well be all out attack, going after Andy's second serve and looking to run around his backhand to hit aggressive forehands very early in the point.
Andy will want to dictate the points, serving well enough to keep James at bay, and looking to test Blake's backhand with both high and very low balls. He'll need to keep a good length or at least keep the ball low if he is deliberately playing short. Taking Blake wide is another smart play that could work for Andy.
If Andy can serve intelligently I think he has a good chance of winning in these conditions. But his serve is often so poor that his opponent doesn't have to do much to win.