There's still time for Jake Paul to reconsider ... 'cause Mike Tyson just shared footage of his first day of training for their upcoming fight -- and the dude looks like he's still packing a ton of power!!

The 57-year-old boxing legend proved his hands haven't slowed down all that much on Wednesday ... hitting the mitts with his coach Rafael Cordeiro.

Tyson also went shirtless to show off his impressive physique ... saying the fun is just getting started, but it's clear he's taking this fight seriously.

The big question when the fight was announced was if Iron Mike -- one of the most feared fighters to ever wear a pair of gloves -- still had it ... and it's safe to say we have answers.

Of course, former UFC champ Chuck Liddell pointed out to TMZ Sports last week that power is the last thing to go in an aging fighter -- and once Tyson makes contact with Paul, the fight will be over.

As for Jake, he's up to the challenge ahead ... and addressed rumors of tag teaming with Logan in the event and the use of headgear.

The Problem Child said it's all BS ... and anyone believing the "reports" is straight-up too dumb to be allowed to vote or drive.