Are you talking about when she lost to Craybas? If so she was just out of match play and not in good shape. Serena's game is extremely high risk so she can only get away with so much if she wants to win matches. If she won she may have played herself into the tournament like Venus did. Although they would have met in R4 so that's another issue. The motivation to win may not have been so high knowing her sister was waiting in the next round.