I am not a fan of on court coaching, but to be able to hear Petrova's coach say "You cannot pressure her [in the rallies] because she plays slow. She plays slow. She plays too slow." was very inspiring.
Wozniacki's problem is that she simply cannot be aggressive on the forehand side. 9/10 times any time she tries to be aggressive on that wing even on a neutral ball, she makes an error. The technique is a complete slap across the ball.