Without his powers, Thor is still a massively powerful human.
Without his shield and SSS, Steve is a bean pole, albeit one with skill. Still his skill advantage, while great, isn't going to be enough against a guy physically superior to typical humans with enough skill and competence to be able to hit Steve at least once, which is all that it will take.
Thor without his powers is still a freaking god from another dimension and weighs alot more than Cap he's massive and probably weighs 2 times more than a normal human his size
He does retain his skill. In the Super Soldier mini, without the serume, Steve managed to beat a couple of roided out henchmen thanks to quick acting and turning their weapons against them.
In a weaponless scenario against a depowered Thor who is still many times bigger and stronger than him and has more than enough to skill to hit Steve, it's not going to matter much.
Assuming he does lose his Asgardian mass in favor for human mass, he'd be well over 220 lbs of pure muscle. Without his godly strength, Thor's still at the very least peak human in terms of physicality.
Except Cap without SSS fought the sisters of Sin. One of them was quite big and was hitting him, she was also shown smashing some metal chair with her fists. These guys (gals) were trained by the Red Skull. Hell at one point he was strapped to a table and still managed to dodge one of them. He also couldn't see properly.
I might put it in the Cap respect thread.
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To Asgardians, being depowered and being mortal are two different things from what I've seen. Stripped of his power, Thor can still hang in a fist fight with Thing, beat down Grog the God Crusher etc. When reduced to a mortal form, he's still peak human or thereabouts because he's in excellent shape but that's it.
Depowered Thor takes it in a stomp. Reduced to a mortal, Thor should still win. Captain America is undoubtedly a more skilled fighter but the gap isn't so large that it can overcome even these odds. And yes, I know, he's Captain America.
Except I don't think Thor would easily beat them. We have seen Thor struggle against canon fodder and these gals were special because they were created by the Red Skull himself, they weren't just nobody. As I said one of them was quite strong, she probably had comparable strength to Thor (powerless of course).
__________________ Watch what people are cynical about, and one can often discover what they lack.
- General George Patton Jr
I think the OP meant no powers at all for Thor including super strength, durability speed etc.... Hence, why he stated these two "Mortals".
Still, it's not easy to call. But Steve (without theSuper-Soldier Serum) did take down several guards who were juiced up on the Super-Soldier Serum with skill.
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Just because he is a long lived warrior doesn't mean he has greater. Captain has masterd a alien form of martial arts and has used it to beaten dudes that has mastered it for years. Cap learns and adapts fighting techniques and tactics like nobody's business.
If things were all equal except skill. Cap would beat Thor without a doubt. Thats why we are talking about Thor in mortal form v.s. Cap without the SS, makes things more even.