Look, her serve is what keeps her winning. She is also strong on groundstrokes but because she cannot move as well (she is no young sprung chicken) she relies on the serve to hold and her powerful groundstrokes to break.
But Sloane gave Serena as much pace as she received. She absorbed Serena's power better than I've ever seen Azarenka or Sharapove do. Moreover, Serena's service game was way off. She just could not get her first serves in, let alone ace her way through.
That may have entirely been due to her ankle and back spasm, but what I do know is that Sloane could have lost the match despite the serve being off. Serena had the match on her racket but Sloane broke back, and so unraveled Serena's nerve and pressure on having to hold to keep in the match.
Oh, well. All is not lost here but this should be a wake up call to Serena to stick to singles like Aza and Maria. The doubles play is wearing her thin.