The realization of stable monolayers from 2D van der Waals (vdW) solids has fueled the search for exfoliable crystals that possess even lower dimensionalities. To this end, 1D and quasi-1D (q-1D) vdW crystals comprised of weakly-bound chains with sub-nanometer thicknesses have been discovered and demonstrated to exhibit nascent physics in the bulk. Although established micromechanical and liquid phase exfoliation methods have been applied to access single isolated chains from bulk 1D and q-1D vdW crystals, the existence of interchain vdW interactions with non-equivalent strengths has greatly hindered the ability to achieve uniform single isolated chains. Here, we report that encapsulation of the model q-1D vdW crystal, Sb2Se3, within single-walled carbon nanotubes (CNTs) circumvents the relatively stronger vdW interactions between the chains and allows for the isolation of single chains with structural integrity. Comprehensive high-resolution transmission electron microscopy and selected area electron diffraction studies of the [email protected] heterostructure revealed that the structure of the [Sb4Se6]n chain is preserved, enabling us to systematically probe the size-dependent properties of Sb2Se3 from the bulk down to a single chain. We then show that ensembles of the [Sb4Se6]n chains within CNTs display Raman confinement effects and an emergent band-like absorption onset around 600 nm, suggesting a strong blue shift of the near-infrared band gap of Sb2Se3 into the visible range upon encapsulation. First-principles density functional theory calculations further provided a qualitative insight on the structures and interactions that could manifest in the [email protected] heterostructure. From these calculations, spatial visualization of the electron density difference map of the heterostructure indicated the possibility of electron donation from the host CNT to the guest [Sb4Se6]n chain. Altogether, this model system demonstrates that 1D and q-1D vdW crystals with strongly anisotropic vdW interactions can be precisely studied by encapsulation within carbon nanotubes with suitable diameters, thereby opening opportunities in understanding dimension-dependent properties of a plethora of emergent vdW solids at or approaching the sub-nanometer regime.