Fighting the Future.

Genkishi is the holder of the Millefiore's fake Mist Ring and he's the one who betrayed the Giglio Nero family, his family, to the Gesso. He's a swordsman, like Yamamoto and Squalo but unlike the two of them he uses illusions and mist flames to back up his skills. He considers himself to be the best swordsman in the world. He is also the 'extra' chapter fight on Squalo's 'Path to Becoming the Sword Emperor' DVDs. Squalo didn't actually believe that Genkishi was fighting him fully, so didn't count him towards his 100 matches. Yamamoto was also given advice on what to do if he ever faced an enemy like Genkishi.

"When you aim for the top, you will sometimes face an opponent who will frighten you to the core of your being. To think you're afraid is not a bad thing. Rather, it's an essential ability that lets you sense your life is in danger. It would be wise to choose not to fight such an opponent. But, as a swordsman there are also fights you cannot avoid. When such a time comes do you know what you have to do? YOU HAVE TO WIN! WHATEVER IT TAKES YOU HAVE TO WIN! There's nothing to gain in defeat. Only victory has it's rewards, wasn't it the same for that ring? Ch. 199, Pg. 16-18.

Genkishi believes he is far superior to Yamamoto and attempts to give Yamamoto an advantage by filling their fighting ground with fake rain to increase the abilities of Yamamoto's Rain Flames and by fighting Yamamoto using just his bare hands. Yamamoto doesn't appreciate this 'respect' and plans to force Genkishi to draw his sword no matter what he does. Yamamoto however also recognizes the strength that Genkishi has and is stunned by his 'killer instincts' believing that they're even stronger than Reborn's.

At the start of their fight Genkishi remarks that Yamamoto's sword style is 'childish' just as he felt Squalo's was when they fought. Using only three of his forms in the Shigure Souen Style (the first, second, and eleventh) Yamamoto is actually able to force Genkishi to draw his sword. Yamamoto who, like the rest of the guardians (except Mukuro) has never had much abilities to avoid or recognize illusions is able to fight Genkishi by recognizing the illusions he uses right before they hit him, unfortunately, he is not able to track the real Genkishi from the illusions. So, while Yamamoto has improved with Reborn he still has trouble with some mist techniques.

Genkishi does prove his skill because his sheer might is able to crack Yamamoto's Shigure Kintoki. Yamamoto tells Genkishi the truth that both he and Squalo realize that Genkishi is stronger but for the sake of the Vongola they cannot afford to lose. Squalo's 'win' against Genkishi was used for Yamamoto to one day fight him and find his weakness, well, perhaps not necessarily for Yamamoto but for the next person from the Vongola who would fight him. Squalo realized that when in his fight he broke Genkishi's sword, that he didn't really break it but instead was fooled by Genkishi's super powerful illusions that distorted all five of his senses and even cameras and machines.

This makes Yamamoto realize that Genkishi never actually made a crack in Yamamoto's sword but instead used his illusions to try and make Yamamoto believe that he actually began to break down the sword. By relying on his instincts Yamamoto is able to see through Genkishi's ploy. Yamamoto's constant practice with his blade makes him aware of it's weight so that if it changed even 0.01g of metal broke off he'd know. Yamamoto shocks Genkishi by telling him this and temporarily paralyzes Genkishi using an attack that he learned from Squalo and made his own. Genkishi recovers quickly however and is able to defeat Yamamoto by forcing him into a wall, quite literally the steel wall box weapon he has.

Yamamoto is defeated by him however when you considered what Yamamoto gains from the fight and the fact that both the older Hibari, younger Hibari, and Tsuna all fight him before he truly goes down, Yamamoto didn't do all that bad.