What happened to Serena during Wimbledon Doubles?

On ESPNW, Martina Navratilova had an interesting take on what happened during Serena and Venus Williams' doubles match yesterday. 

I now wonder if Serena is suffering burn-out. I don't know about all the physiological signs she was exhibiting in her doubles match (see video), but her singles loss against Cornet was clearly not Serena at her best - which is not to take anything away from Cornet or anyone else who has beaten Serena this year.

Think about what Serena accomplished last year... It was arguably the greatest year in her career. She won over $12 million in prize earnings - not even counting endorsements (which are still less than Sharapova, but that's another story). Given what a great year Serena had last year, maybe she just needed a break. She is 32 years old after all (I never thought I would think of that as young!:-) During much of her early career, Serena did not play as intensely as she did over the past 2 years. After she lost to Garbine Muguruza at the French, she said she would go out and work 5 times as hard so she would never lose again. Maybe she needs a break. Maybe that is what her body is telling her now.

And then there is the issue of why she even went out onto the court if she was in such a state of disorientation! I can think of a good reason for that - remember Indian Wells in 2001? When Venus withdrew from the tournament, Serena heard the vitriolic booing of fans who thought her win by default over Venus was "fixed" by her father. The suggestion that she just could have defaulted fails to take into account what has happened in the past when either she or Venus defaulted. That's why I think Venus was proud of Serena for at least trying to play!

What do you think?