Upconversion nanocrystals convert infrared radiation to visible luminescence, and are promising for applications in biodetection^,,, bioimaging^,,,, solar cells^,, and three-dimensional display technologies^,,. Although the design of suitable nanocrystals has improved the performance of upconversion nanocrystals^,,,, their emission brightness is limited by the low doping concentration of activator ions needed to avoid the luminescence quenching that occurs at high concentrations^,. Here, we demonstrate that high excitation irradiance can alleviate concentration quenching in upconversion luminescence when combined with higher activator concentration, which can be increased from 0.5 mol% to 8 mol% Tm³⁺ in NaYF₄. This leads to significantly enhanced luminescence signals, by up to a factor of 70. By using such bright nanocrystals, we demonstrate remote tracking of a single nanocrystal with a microstructured optical-fibre dip sensor. This represents a sensitivity improvement of three orders of magnitude over benchmark nanocrystals such as quantum dots.