I watched a few games and noticed that Serena's serve was lacking its usual pace. She seemed to be spinning more of her first serves and only occasionally going for a big one. This was confirmed by simply watching the speed meter in the corner of the court. While still hard by many players' standards, her numbers were noticeably slower than what I have grown used to seeing. Often times when Serena really uncorks one, she will reach nearly 200 km/H. Once in a while I have seen her get to 200 or 201. Regardless, her first serve will often be in the 190s unless she's going out wide in the deuce court. Today I was seeing 160s. At this level, it's a big difference.
Just now, I read on ESPN that she has been dealing with a back problem.
Ivonovic looked to be playing well from the part of the match I saw. Kudos to her. Serena's serve was also not up to its usual devastating standard.