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.