Niš is a small city in Southern Serbia with 250.000 inhabitants. It is a nice place to stay overnight. It is on the main high-way between Belgrade and Skopje, as well as between Belagrade and Sofia. If you're coming from Belgrade, on your way to Istanbul, it might be easier though to hitch straight to Sofia, and to get a ride towards Sofia at the last toll-station just before Nis, as it is only 190km further.
The road from Nis to Sofia is partly high-way. It should be possible to get a ride from Nis towards the border of Serbia with Bulgaria. If you do not get one direct ride, be aware that there are not that many good spots on this road.
Route to Istanbul
User:Robino got a lift from his host in Nis to the highway around Nis. There is generally not much traffic in the morning, and he is not aware of any good spots or how to get there.
User:jurgen walked 3 km down the main road east from central Nis, and started hitching there. It worked fine to get out of the city on to some local roads, from where he got another easy ride towards the border and Sofia.