I rewatched a clip of Serena's service game several times this morning - clearly she is disoriented, her hand eye coordination is way off. Her gait seems ok, but she is shying away from light in her eyes. As I have been dangerously ill with virus' in the past, I can attest that this is not unusual especially in strong light which can cause headaches to intensify.
I am prepared to give her the benefit of the doubt here as there is no evidence otherwise, and I cannot for the life of me believe that someone who has been through life threatening illness and worked her way back to the top would jeopardize all that by taking any substance that would impair her abilities, especially before a world wide audience. I understand that there have been upsets in her personal life, but again, there is no proof to show that the impairment is related to that as well.
I hope that whatever there is wrong, rightens itself, and we see her at the US Open.