I think it went like this:
FS underestimated him, so he didn't play that serious at the start.
QuickZ had a good day, so he sort of pwnd him at the start.
It took a while before FS started playing very serious, more defensive.
QuickZ was like "OMG WHATS HAPPENING HERE" and lost his concentration.
That probably is the reason why FS got such a huge comeback, and why Quick wasn't able to "finish him off"