Portgas D. Ace was defeated in the battle with Blackbeard and then held in the deepest level of Impel Down, bound with Seastone handcuffs the entire time. That should have spent what ever strength he might be holding. He died protecting Luffy. That strike was directed at Luffy. He might have left in well understanding that he would die.

How Strong Was Portgas D Ace? – Explained

Devil Fruit: Mera Mera no Mi

Ace had excellent control over his Devil Fruit, evident in the versatility of his attacks. Additionally, when Ace was on Drum Island, the townspeople told the Mugiwara pirates that it didn’t snow that whole day. This was unusual since Drum Island is known for its daily snowfall, indicating that Ace might have awakened his Devil Fruit abilities.

Physical Strength:

Van Augur states that still without his devil fruit Ace’s physical capabilities are formidable as expected from a Whitebeard Commander. Ace clashed off Jinbe for 5 days which is exceptionally physically powerful, showing Ace has absurd levels of stamina, only above tiers have been established to combat for days like Admirals and Yonkos etc despite Luffy’s prolonged fight has only been 12 hours or so.

Haki:

It’s been shown via the Ace novels that he could handle Armament Haki, which he awoke mid-fight next to a Vice-Admiral & it was powerful than the Vice-Admiral’s own Haki following only after awakening it, there’s no knowing how much stronger it would have gone after three years of New World fights. We now know he had Conqueror’s Haki, which he awakened as a child. In fact, his Haki was so dominant in the Ace novel that when he stepped onto the snow-covered island where Yonko Shanks was residing, the Red-Haired Yonko remarked, “His Haki is like fire, which is why the snow is melting on the island.”

Portgas D Ace Vs Admiral Sakazuki:

Ace was regularly beaten in Impel Down and given little to no food or water, so he was not at his full potential.
Given all the facts mentioned above, it’s safe to say that Ace is not far behind the so-called Admirals—he’s powerful enough to confront and injure an Admiral, but not to defeat one. That’s the reason Ace had to be finished off, it’s because he’s extremely more capable than Luffy & he would have become the Pirate King if he touched his potential.
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