I understand that Williams fans will be delusional because their emotional well being rests on the lie (formed when Serena had a brief, 1.5 year period of dominance in which it was almost true) that no one is good enough to beat Serena other than Serena herself. However, an unbiased analysis with its basis in facts (i.e. match statistics) reveals a far different picture.
What really happened is that Jankovic neutralized Serena's serve because she got most of them back really deep without much pace, allowing Jankovic to give Serena few free points on serve and also to get back to the center of the court. It was brilliant. From there, Jankovic kept the ball within a foot of the line on almost every shot, and did not miss. Jankovic had five more winners than UFEs. For a match of this caliber, that is very very solid indeed. But what's more striking, Jelena won 3 out of every 4 points when Serena put in a second serve.
Serena's second delivery was rather slow, but so was Jelena's, yet Jelena managed to win 50% of second serve points. Jelena also capitalized on her break point opportunities--winning almost 50% of them. Against a player as mentally tough and with as big a serve as Serena, that's incredible and speaks volumes about Jelena's improved mental toughness.
But the biggest statistic that reveals that Serena was not nearly as big a factor as Jelena in this match's result, is the fact that Serena hit 34 winners to 36 UFEs--against a player as fast as Jankovic. Serena has won matches against top players with a -10 and even -15 numbers before. Granted, anytime a player is in the negatives, that does mean she's not zoning or playing her best, but anywhere within 5 of neutral shows that a player is playing well enough to win. Jelena was +5, so she was playing very well, but that was a bigger factor.
I think the truth is that Jelena's form of return takes away Serena's biggest advantage, and will until Serena becomes comfortable with serving and volleying.