Remember, Thanos has sufficient fear of Warlock as an opponent, that he went to all the trouble of allying himself with Adam in his war against the Magus's Universal Church of Truth a) in hopes that Warlock might remove the Magus from the board for him, and b) to absorb enough residual energy from the Soul Gem to power his star killing machine without trying to take it from Adam.
I's not that Warlock is more personally powerful than Thanos, he is simply more consistantly effective.