Serena got yellow card because She screamed c'mon when action was on and the judge said that it was unfair and might prevent Stosur to do next action.
It is interesting because Sharapova's loud screams when action is on always prevent Other Players but She never got yellow card.
thank you, i havve seen it now on youtube. i myself play tennis. and i have to say, that there is really a policy against the williams sisters. i cannot believe that sharapova or (theres is another girls who grudges very loud) do not get a warning or a yellow card. but this referee had many problems with williams as i can remember ! sad for serena but she played not her best.