Focusing on the social aspect of sustainability, this article provides insight into the process of spatial and functional reconsideration of open public spaces in two selected super-blocks in Belgrade, Serbia. Although their spatial typology is similar, one of them was created during the 1960s in New Belgrade, a new administrative center of the city based on the principles of functionalism, while the other one was built during the 1970s, as a part of urban reconstruction conducted in the central areas of Belgrade (Vračar municipality). The beginning of the 21st century has brought new challenges to open public spaces, reflecting the post-transitional changes of the Serbian socio-economic context, as well as the contemporary urban needs of inhabitants. Consequently, both blocks have developed new gathering places for their local communities, although applying two different approaches, spontaneous/informal (New Belgrade) and formal (Vračar). Considering the specificities of both initiatives and the relationship between local communities and the open public spaces of super-blocks, the comparative analysis is conducted in order to identify the occurring social, spatial, and functional modifications, and the achieved level of social sustainability.