It's a shot to play in that situation. A good play as far as I can see and no reason to condemn it just because he missed. The FH drop shot is something Federer almost never does, he is expanding his reportoire.
Meanwhile, great play by Federer to reach duece.
Fed's got the greatest forehand in the world, the Ad court was wide open, he didn't need to go for a dropshot in that case, a cross-court forehand would've guaranteed him the point. A Slam SF in not the place to show off your repertoire and experiment shots you don't normally go for.
Meanwhile, a great forehand winner dtl by Fed, 4-1*