All I can say is wow!!. I picked Serena but knew that Gabby would be a difficult match. This was especially true after I saw how Serena played in the round. Although the score-line showed an easy match, she was too tense and way too many UFE's. Martina N. said it best, at Serena's stage in her tennis career, it is not always about the physical side of the game; mentally you are aware of your place in history and realize that you have so much more to lose. Serena still has the physical weapons, but her emotions/mental will continue to sometimes fail her at the big moments.
That said, congrats to Gabby, may she go far. The field is now wide-opened for someone to prehaps win their first major or for Maria to win her fifth.