That's probably one of the reasons of her defeats against Maria, especially this yesterday's one.
With her 4-0 lead, even though she was in pain, she had the match won because Maria was totally out. Serena couldn't serve but she could break Maria twice more and it would be done.
I think there was something psychological there, Serena couldn't do something that would "hurt" Sharapova so her subconscious made her unable to play some tennis anymore. That's a new tennis-concept: the "Love-choking".