Roronoa Zoro is one of the main characters in One Piece who sails with Luffy, the protagonist of the story, as one of the most important members of his crew. Zoro is a master swordsman and one of the strongest characters in the series right now.
He aims to become the greatest swordsman of his era and, to do this, Zoro has to reach unprecedented levels of power. During his journey across the seas, Zoro has encountered several strong fighters who have pushed him beyond all his limits. Out of the many that he has fought, only a few have tested Zoro both physically and mentally.
Note: This list isn't based on the strength of Zoro's opponents but on how difficult the fights were for him when he fought them.
11 Zoro Vs. Hatchan
Zoro fought his battle against Hatchan during the Arlong Park arc of One Piece. While Hatchan was far from the strongest that he has faced, this fight saw Zoro carry wounds from his clash against Mihawk.
This nerf made this fight incredibly hard for Zoro and made Hachi one of the strongest that he has faced on his journey so far. In the end, Zoro managed to defeat him despite his injuries.
10 Zoro Vs. Ryuma
Zoro clashed against Ryuma during the Thriller Bark arc of One Piece. In order to win back Brook's shadow, Zoro challenged the samurai Zombie to a fight that ended rather quickly due to the incredible intensity that it showcased.
This fight ended with Zoro slicing through Ryuma with Hiryu Kaen and earning his respect in the process. As a reward, he also got the sword Shusui.
9 Zoro Vs. Kaku
Kaku was a member of the CP9 who played the role of Zoro's main opponent during the Enies Lobby arc. With his newly acquired powers of the Ushi Ushi no Mi, Model: Giraffe, Kaku proved to be quite a challenge for Zoro.
In fact, even at his best, Zoro struggled against Kaku to quite some extent. Zoro had quite a lot of trouble dealing with his unique attacks and his Rokushiki abilities. This fight required him to awaken and tap into the powers of Asura and level up during the fight to defeat his adversary. Despite everything, Zoro had enough strength to continue fighting afterwards too.
8 Zoro Vs. Kuina
Kuina is, by far, the weakest person on this list that Zoro has fought and that is down to the fact that she was a child when they clashed, however, at the time, she was the strongest that he had entered combat against. Kuina fought Zoro 2001 times and defeated him every single time with superior swordplay skills.
While 2000 of those fights were fought without an edged blade, the last one was fought with real blades and, once again, she ended up winning. She is one of his toughest opponents, evident from the fact that he wasn't able to defeat her even once.
7 Zoro Vs. Pacifista
On Sabaody Archipelago, Roronoa Zoro and the Straw Hat Pirates came face to face with a Pacifista. This fight was incredibly hard for the Straw Hat Pirates and not even the Monster Trio was enough to overcome it.
In the end, it took the collective effort of the entire pirate crew to subdue this Pacifista. Zoro alone struggled against it to quite a degree and, today, this remains to be one of his toughest fights.
6 Zoro Vs Mr. 1
Zoro's fight against Mr. 1 was incredibly brutal for him as it gave him a reality check of how far he was from his goal of becoming the greatest swordsman in the world. Mr. 1 had a steel body, thanks to his Supa Supa no Mi.
At the time, Zoro did not know how to cut steel and, thus, he suffered heavily in this fight. In fact, Zoro was left in a near-death state while fighting against him. In the end, he had to evolve while fighting and learned how to cut steel in order to surpass the fighting prowess of Mr. 1.
5 Zoro Vs Bartholomew Kuma
Zoro clashed against the Warlord of the Sea, Bartholomew Kuma, during the Thriller bark arc of One Piece. Shortly after the fall of Gecko Moria, Kuma showed up to kill Luffy and take his head to the World Government. Before he could do that, however, he decided to exterminate the entire crew, starting with Zoro.
Zoro took the challenge head-on and the two fought in a short-lived battle where Zoro immediately realized that he was no match for the Shichibukai. In the end, Zoro ended up offering his head instead of Luffy and earned Kuma's respect. Despite everything, he lived but was ordered to take all of Luffy's pain.
4 Zoro Vs. King
Zoro fought against King the Wildfire in Wano shortly after being in a life-and-death battle against two Emperors of the Sea. After taking a Mink drug that restored him to full health for a short while, Zoro engaged King in combat and suffered for the vast majority of it.
After being heavily injured, Zoro mastered Enma and learned how to channel Conqueror's Haki. Thanks to this ability, he developed a new fighting style, called King Of Hell: Three Sword Style. Eventually, Zoro defeated King with his newfound abilities.
3 Zoro Vs. Enel
In the Skypiea arc, Zoro ended up fighting against Enel, even though he was not strong enough to take him on and combat. As expected, the fight was short-lived and he ended up losing this battle within a matter of a few minutes.
Enel's lightning powers were enough to electrify Zoro and knock him out completely. Even with the help of other fighters around him, Zoro stood absolutely no chance and was defeated.
2 Zoro Vs Mihawk
Quite easily one of the toughest battles that Zoro has been in is the one that he fought against Dracule Mihawk, the man who currently holds the title of the World's Strongest Swordsman. Zoro challenged him during the Baratie arc and found out the gulf of power between him and the World's Strongest Swordsman.
In the end, he admitted his defeat and received a scar that runs across his chest as a mark of respect from Mihawk. Undeterred, Zoro vowed to never lose again.
1 Zoro And Worst Generation Vs Kaido And Big Mom
Quite easily, the most difficult fight that Zoro has been in so far is the one that he fought against two Emperors of the Sea on Onigashima in the New World. Both Big Mom and Kaido made life hell for the Worst Generation on the roof of Onigashima and Zoro, who ended up losing this fight, was extremely impressive here nonetheless.
Zoro ended up holding Hakai, a powerful Haki blast by the two Emperors, momentarily before being teleported by Law. This attack saw countless bones in his body shatter and despite that, he was able to fight and even scar Kaido. Sadly, Kaido went on to knock him out with a powerful Thunder Bagua shortly afterwards. Had it not been for the doctors around him, Zoro would certainly have died. Nonetheless, he learned a lot from this fight and after being healed with a powerful Mink drug, he went on to fight against King the Wildfire.