People keep saying Thanos can't lose to Superman because he's a lower tier character. If full powers were in effect, I'd accept that a little more. I'd be giving Thanos the edge if all powers were in place, and just give Supes a couple wins...
But in hand to hand, Supes just dominates him. That's his playground. Supes has the lifting feats, the punching inanimate objects feats, the punching other characters feats, the superspeed feats, the flying feats. On and on, he has every physical edge save durability, which is relative given the stuff they've lived through.
You can find one of the smartest guys in the world, and they're still not going to beat the chess champion in a chess match.