Um no, Proteus would be a terrible match up against Thanos. Thanos with vast experience with Cosmic Cubes, IG, and the HOTU, has a WAY better lock on how Proteus' powers work than even Proteus does in reality warping. First of all Thanos was depicted in Thanos Quest as having no trouble with reality shifts. I believe his line was "Reality has a minimal effect on my being." In addition to that and having been in all sorts of realities, Thanos possessed the Reality gem which certainly provided him with some keen insights into how reality warping works. So having that knowledge. He would understand Proteus' powers better than Proteus.
Originally posted by TheLordofMurder
The OP didnt state that there was prep available to either side; its a fallacy on your part to assume otherwise...Sans prep, Proteus dominates Thanos; there is nothing Thanos can do to a non-jobbing PIS'ess Proteus...