Serena is far from mentally weak, but she sure seemed rattled in this match, even before the injury. In fact their match up in Hobary or wherever was fairly tense too.
I don't doubt her back tightened up but when the 3rd set began Serena was playing her normal game. Maybe not at 100% but Sloan was scrambling, playing great D and being aggressive.
I think Serena could have and "should have" won this match but Sloan took it, injury or not.
And seriously, Williams saying these were the worst 2 weeks or whatever...until this match she had barely dropped 2 games in a set.