It looks quite windy, she has been doing it a often doing the tournament. I watched her first match and then her QF. Sometimes she hits flat and hard, and most of the time it means easy service holds, then some times she just spins her first serve in. I guess she doesn't feel the pressure from Rezai like she does against the top 50 players etc.
You're asking me to go into the head of Dementieva? Not even God himself could do that I also think it saves her makiing lots of DF's, just put the first serve it and let the ground strokes do all the work.