Another major factor is the inconsistency of her serve. She served 8 aces against Arn and Many unreturnables, but today she just span a lot of her first serves in. It just wasn't firing when she really needed it, whereas someone like Serverena pretty much knows how that she is going to serve huge from one tournament to the next.
But fitness has actually cost her some matches. See the Paris indoor semi vs Chakvetadze a few years ago where she was cruising a set and a break up and then injured her back because of her conditioning and didn't win a game. To this day, I believe she won have won that tier 2 title had she been in better shape at the time
but we all remember that the first half of 08 (where she was giving out top 10 wins left, right and centre) was probably the worst tennis of her career, and that was primarily due to her fitness.