Well, i was very interested in knowing where was the tennis court in Tsarskoe, after looking many photographs i´m quite sure it was in the courtyard inside the Feodorovsky Gorodok, just behind the building that housed the banqueting hall.
I hope you understand what i mean with the photographs:
Photo taken by Maria. Notice the wall and the part of the building with the red arrow.
In this painting from 1915 notice the same part of the building seen from outside the wall that surrounded the courtyard (Also see that several of the soldiers are carrying rackets) :
And finally this one i took last summer. It shows the place where the tennis court stood but from a slightly different point of view, more to the right, than the first photo. The wall is still there but it´s hardly noticeable because it has lost the white paint.