Re: Federers second serve!
Faster surfaces (indoor) does not respond to topspin as much. Also Federer isn't trying to expend as much energy (not just serve, but shots too) in these post-USOpen tournaments.
You know when his second serve was awesome good? Australian Open (responds to topspin well). He had a horrible first serve percentage there, but his second serve wasn't attackable so he started the rallies in a good position.