No, reaching over the net with your racket is not allowed in tennis. You need the ball to have reached your side of the court first. The only exception to this is if the ball lands on your side and either due to the spin on the ball or the wind, the ball bounces back over the net and onto the other side of the court.
Height and position: The net should be suspended by a cord or a metal cable that is attached to two posts called net posts. These net posts are at a height of 3.5 ft (1.07 m). And the space between them should be totally filled by the net. At the center of the court, the net should be at a height of 3 ft (0.914 m).
Net: divides the court in half; Net posts: holds up the net on either side of the court; Baseline: lines at the end of the court; Center mark: a small line at the center of the baseline; Singles sideline: inner lines at the sides of the court; Doubles sideline: outer lines at the sides of the court; Doubles alley: area between the singles and doubles sidelines
When in play, the player cannot touch the net or net post, or cross over onto the opponent’s side of the court. If they do, this player will lose the point. A player can only hit the ball once before returning it to the other side of the net. The boundary lines of the court are considered part of the court.
You are not allowed to touch the net with anything until the point is over in a game of tennis. This is duly covered in ITF rule 24g. In the rule, it is even stated that if any part of your body, your item of clothing or even your racket touches the net or the net posts while the ball is still in play, then the point is awarded to your opponent.
(13) A player can’t touch the net, with anything, until the point is over. (12) A player can’t hit the ball into anything other then the court or net, before it lands.
Your racquet can cross the net as long as it never touches it. This does not mean you can just start reaching across the net to hit balls before they come to your side. Before you can hit a return, the ball must first cross over the net to your side of the court. (ITF Rule 24)
The rule is that you can't touch the net until the ball bounces twice - i.e., you can't touch the net until the point is over. If that's the spirit of the rule, why not give the point to Schiavone anyway because Kuznetsova had no play on the ball and the point was over when the ball passed Kuznetsova for a winner?