Warning: contains spoilers for Guardians of the Galaxy #7!Marvel just casually confirmed that a surprising hero is on the same strength level as the Hulk - right as they ascend to an even more powerful ultimate form. While fans knew this hero - familiar to many from their major role in the MCU - was strong, few would have considered him an equal of the Green Goliath.
In Guardians of the Galaxy #7 - by Jackson Lanzing, Collin Kelly, Kev Walker, Matt Hollingsworth and Cory Petit - Wiccan and Hulkling venture into Grootspace, the new form of Groot after bonding with the rest of the Guardians of the Galaxy. Touching down, Wiccan notes that "Groot was strong enough to 1v1 Hulk" even before he transformed.
Groot has long been the Guardians' muscle, going toe-to-toe with super-strength threats including Thanos, Magus, and Gladiator. However, due to his relatively few encounters with Hulk, it's often been difficult to compare them.

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Hulk & Groot's Original Battle Confirmed They're Equals
Hulk and Groot did clash in Len Wein, Chris Claremont and Sal Buscema's The Incredible Hulk Annual #5 - at least after a fashion. In the 1976 issue, the alien being Xemnu recreates several major threats to Earth, turning them into a gauntlet for Hulk. Accordingly, Hulk fights the original Groot introduced in Tales to Astonish #13. The fight is remarkably close - Hulk's blows fail to knock Groot down (since he's rooted to the ground), and the Flora Colossus is able to both send Hulk flying with a punch and restrain him with vines and branches. Hulk triumphs, but it's more thanks to his tactics than superior strength, first uprooting Groot by attacking the ground, then using a boulder to fend off his powers and smash him to pieces.
Marvel has been inconsistent about whether this Groot is the same one fans know from Guardians of the Galaxy comics - while this was originally the case, different stories have vacillated on the details. Either way, however, the issue confirms that of Groot's species can indeed face off with Hulk in of pure strength. Groot's race have also been hinted as the direct offspring of the Elder God Gaea (also Thor's biological mother), explaining why their potential is so extreme.
If Groot Wasn't Hulk's Equal Before, He Is Now
Of course, Wiccan's second point is more relevant - whatever Groot was capable of as a single individual has now been multiplied by thousands, as his spores give him direct control over not just an entire planet, but an entire sector of space. Having absorbed the powers of the first Flora Colossus, Groot has now become something very close to a god. Thankfully, as threatening as he seems, he's a benevolent god - Groot is playing the long game to protect and bring about the rebirth of everyone in Grootspace. However, in order to do it, he'll need to keep them safe from the various cosmic powers who see his new form as nothing but a threat.
Groot may have the strength of the Hulk, but in his new form, he's also living the same life - an essentially benevolent being targeted for endless violence by those who fear his strength.
Guardians of the Galaxy #7 is available now from Marvel Comics.