Ah there is indeed a Topsstrasse there...
The flea market is in Mauerpark, which is exactly west of the stadium. Here is a view from the Mauerpark - you can see the lights from the football stadium right behind the hill:
To make it simple, the flea market is about 100 metres (?) behind where this picture was taken (that is in the opposite direction! i'm not sure i'm very clear here).