We show exciting research opportunities when the concept of molecular self-assembly meets photon upconversion (UC) based on triplet-triplet annihilation (TTA). Previous examples of TTA-based UC utilized the diffusion and collision of sensitizer and emitter molecules in some dispersion media, however, there are limitations such as quenching by oxygen and low diffusion rate in the solvent-free state. In this context, we propose a mechanistic paradigm shift from the molecular diffusion in dispersed systems to the energy migration in condensed systems. In dense dye assemblies, triplet excitons can efficiently migrate and annihilate, and oxygen penetration can be reduced. For the proof of concept, we show the first observations of TTA-UC in a solvent-free molecular liquid and supramolecular gels. The liquid having an emitter moiety was doped with a triplet sensitizer. Significantly, the obtained liquid shows UC emission even in the aerated condition with high quantum yield. We also show in-air TTA-UC by dense accumulation of dye molecules in supramolecular gel fibers. By just adding donor and acceptor molecules in the gelation process, clear TTA-UC emission was successfully observed in the air-saturated condition. Moreover, the air-stable TTA-UC in supramolecular gel nanofibers was generally observed for a wide combination of donor-acceptor pairs which enabled near IR-to-yellow, red-to-cyan, green-to-blue, and blue-to-UV wavelength conversions.