Re: What happened to Stosur's second serve?
I think it's probably mainly a confidence thing, and tbh since her first set against Safarova in the FC she has only hit about 5 or six DFs in six sets (including her doubles yesterday), so I don't think it's anything dramatic.
In general I think she's going for variety on serve these days anyway, rather than throwing in a huge kicker every time. It stands to reason when you're varying your line, pace, spin, etc. a lot you are more susceptible to hitting DFs