(abridged) A detailed study of the blue supergiant UIT005 (B2-2.5Ia+) in M33 is presented. The results of our quantitative spectral analysis indicate that the star is a very luminous, log(L/Lsun)~5.9 dex, and massive, M~50 Msun, object, showing a very high nitrogen-to-oxygen ratio in its surface (N/O~8, by mass). Based on the derived Mg and Si abundances, we argue that this high N/O ratio cannot be the result of an initial low O content due to its location on the disk of M33, known to present a steep metallicity gradient. In combination with the He abundance, the most plausible interpretation is that UIT005 is in an advanced stage of evolution, showing in its surface N enrichment and O depletion resulting from mixing with CNO processed material from the stellar interior. A comparison with the predictions of current stellar evolutionary models indicates that there are significant discrepancies, in particular with regard to the degree of chemical processing, with the models predicting a much lower degree of O depletion than observed. At the same time, the mass-loss rate derived in our analysis is an order of magnitude lower than the values considered in the evolutionary calculations. Based on a study of the surrounding stellar population and the nearby cluster NGC588, using WFPC2 photometry, we suggest that UIT005 could be in fact a runaway star from this cluster.
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