100% unacceptable. She won her match in straight sets without grunting, so it's obviously not done out of effort put into shots. This is the quote right here that tells me she is using the grunting simply to try and rattle her opponent," Definitely if the matches are going to be tougher, obviously I'm going to start grunting." Personally, if I am a player facing her I would do anything in my power to return the favor. I would take as much time as possible on serves (maybe even give her a quickie serve if she had her hand up), changeovers, call for the trainer when it's not necessary, take bathroom breaks, anything. It really is just something used to gain an advantage over another player, and it's sad to see, especially at Wimbledon.
In her defense, the only thing I could say is that she is clearly being singled out. Other women do this as well, most notably Maria Sharapova. Here is an interesting little video of some points from Sharapova and de Brito. Clearly they are both obnoxiously loud and highly unnecessary.
http://www.youtube.com/watch?v=WVfCcBzM
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I think the WTA needs to step up and do something about it. Really the only two guys who I can think of that do anything like this are Nadal and Ferrer. Those guys go all out every point, so it is very understandable. Even so, it is nowhere near the level of Sharapova and de Brito. Since it clearly is just a tactic used to gain an advantage over an opponent, I think there should be a certain noise level that is tolerated. With today's technology maybe there is a device that a chair umpire could have on them that would beep or something once a certain noise level has been reached, and that device could be turned off an on between points. Women's tennis is basically unwatchable for me, and this is definitely the biggest reason.