If you live in the States and rely on TV as your main source of tennis, you saw, at most three minutes worth of this match. All I know, based on heresay, is that JJ got broken a lot, got pissy a lot, and laughed when Shvedova whiffed on an overhead on break point.
But based on what I saw, here's why I'm worried for Sam tomorrow:
1. JJ's backhand down the line will be able to pull Sam to her forehand side and thus open up her weak backhand.
2. JJ's going to get a lot of balls back, forcing Sam to hit with less margin to get a winner.
3. JJ can handle slice.
4. JJ never goes away. Her ability to compete is totally underrated.
To me, Sam's best still beats JJ's best. Their H2H is 3-1 in JJ's favor, with JJ beating Sam in straights in IW and Sam beating her in Miami (after she had won IW). If Sam can bomb some serves, hold easily, and take some cracks at JJ's puffball serve, this could be quick. But if JJ can lure her into rallies, scramble around, and pressure Sam into making errors, the slippery Serb may be on her way to her first Grand Slam title.