How was Thanos able to defeat Thor in the beginning of Avengers: Infinity War? Did he use the Power Stone?
The whole battle was left off screen so we can't be certain but there are several things that turn the tides to Thanos' side.
He was defeated by Thanos, wielding the Power Stone, and Thanos’s children.
This whole battle is left offscreen, so we’re left to extrapolate from what we are shown. All we know is that they were defeated severely enough that half of the Asgardians were slaughtered (while the other half, with Valkyrie, Korg, and Miek, possibly escaped somehow), Heimdall was near mortally wounded, Thor was so wiped he couldn’t even move on his own, and Loki was restrained, only unharmed likely since he was one of Thanos’s former minions. Not to mention the Asgardian ship Statesman was blown nearly to pieces and on fire, to boot.
Since we see Thanos defeat Hulk, seemingly without much effort - and this without even using the Power Stone, if I remember correctly - this level of destruction doesn’t seem out of character. And if Thanos had used the Power Stone, capable of destroying whole planets, during this battle before Hulk came along, then unfortunately it’s not much of a surprise that he defeated the others so thoroughly. The Statesman, being filled with civilians and largely unarmed due to not being a warcraft, had little chance against him and the Black Order. I’m sure Thor and co. fought their absolute best, but in the end just couldn’t overpower Thanos and his children.
- How was Thor so badly beaten up between the end of Ragnarok and the beginning of Infinity War? - Quora
So, with this the obvious conclusion is that the might of the power stone combined with Thanos' children's power ultimately overthrew Thor and co.
I'd imagine it was because Thor was not at the peak of his power considering what happened in Thor Ragnarok. Hela had destroyed his Mjolnir. And the final battle had left both him and the Hulk weak. The only option they had was to unleash Surtur on Hela (and Asgard). The survivors were reduced to refugees on a spaceship as Asgard was destroyed.
When they run into Thanos, it's not a battle, but a slaughter.
I would more or less guess at the fact that Thor, Loki, Hulk and co had just had a tough battle with Hela, resulting in the demise of Asgard, all the remaining Asgardians were more or less refugees as opposed to soldiers. The ones who could fight were exhausted from what they just did, both physically and emotionally (losing their entire homeworld).
Thor was basically powerless, as I don't think he can call upon his full lightning power whilst in space, and it was basically lambs to the slaughter for Thanos.
Thanos seemed to know the exact location of all the Infinity Stones as well (barring the Soul Stone), so logically, he went to Xandar to collect the power stone as he probably knew Asgard with Odin would be a tough battle to acquire the Space Stone.
Thanos probably watched Surtur destroy Asgard from Sanctuary as he was heading to Asgard next for the Tesseract, then spotted their ship leaving.
It makes no difference, Thanos is wearing his armour at the beginning of Infinity War which makes him much stronger and more powerful, so Thor can't win whether Thanos uses the stones or not.
Most people seem to be unaware that Thanos' armour has this effect, it's like the difference between Iron Man with and without his suit, except that he doesn't really need the armour except for protection against Thor (hence why he was able to kill him at the end of Infinity War as he stopped wearing his armour).