The thing is, is Ace's true strength was never actually revealed. We only got a glimpse of his strength, which came after he spent weeks (Months?) being tortured and restrained in Seastone handcuffs in Impel Down. He had no time to recover or heal, but as soon as he broke free, he was able to cancel out Aokiji's Devil fruit with his own. He's much stronger than people give him credit for.
Last we saw in a flashback is that Ace and Yamato fought to a draw, and this was prior to him joining the Whitebeard Pirates. With every flashback, Oda reveals more and more about Ace's strength.
Oda was actually pretty slick, as outside of Yamato, he only ever showed Ace going up against people who directly cancel out his DF ability. Akainu (Magma) Smoker (Smoke), Aokiji (Ice) and Jinbei (Water). This way he could make Ace as strong as he wants when he finally decides to reveal his actual strength.