Early into Fate/Stay Night, there is a scene where Shirou sees Sakura and another man:
In the Unlimited Blade Works Route, we confirm this other man is Gilgamesh. When Sakura talks to Shirou, she says that the other man was a foreigner and she couldn't understand him. However, we can see from the image that that may not be the case.
Since Gilgamesh has been tainted by Angra Mainyu's corruption of the Holy Grail from the previous war, and Sakura is a "Black Grail" Vessel with a deeper connection to Angra Mainyu than the "White Grail" Vessel (Ilya), did Gilgamesh already know about Sakura?