What does Superman usually open with, I remember him usually going for that melee
Firstly Thanos dominates in close combat. Superman scales above 2.72 Gigatons while Thanos slapped someone who's 3.75 Gigatons. He has the AP and LS advantage, as well as much greater combat experience and feats than Superman, meaning if Superman were to attempt to attack him in close combat, he's gonna get clapped pretty easily. It doesn't help that Thanos has the power stone, meaning he could very much one-shot Superman if tries to use it.
The main wincons I see for Superman here tho is freeze breath, flight and maybe his illusions? he could potentially try to BFR Thanos into space, but that would only be if he could tackle Thanos push him up there, and considering the AP and LS advantage, Thanos could just knock Super before he manages to do that, or if Supes came charging at him. As for freeze breath, Thanos could just use the space stone to shield himself against it, or maybe counter it with the power stone, or try to shoot Superman with it. As for illusions, Thanos could just use the power stone to break them like he did with Dr Strange, and I don't think Superman's illusions can even harm Thanos. Also if Supes ever tries his laser vision, Thanos could try to block with his sword
In conclusion, Thanos takes this with Low to Mid Diff if Supes uses his Hax, but if Superman goes and tries bullrush him, then it's a GG for Supes.
I vote Thanos