Floyd Mayweather Jr. and Ronda Rousey are the reigning king and queen of their respective sports. Mayweather is the pound-for-pound king of boxing, and the WBC and WBA welterweight champion . Rousey is the UFC women's bantamweight champion. Neither has been beaten yet.

These two fighters have been plopped in countless hypothetical matchups that won't be realized. Rousey has said it herself. However, it is worth comparing what two different fighters in two different sports share that make them such dominant champions.

Both Have One Trademark Skill

Mayweather became the king of boxing because of a well-rounded skill set, but his most prominent advantage over his opponents is his famed "shoulder roll" defense. In his younger days, trying to punch Mayweather was like fighting smoke. Clean shots were almost impossible to land.

When backed into a corner Mayweather tends to poke up his left shoulder and allow it to deflect incoming shots, while his right arm protects his body. For most fighters this is a risky proposition; it leaves many holes to attack if the fighter using this defense isn't as quick as a cat.

Fortunately for Mayweather, he's even quicker than that. By being quick and whip-smart, Mayweather's opponents find their punches glancing off his shoulders before he sharply counters them and gets out of harm's way.

For Rousey, her defining skill is undoubtedly her famous armbar. She's finished off nine of her 11 professional opponents via armbar, most recently submitting Cat Zingano, billed as her most difficult test ever, in 14 seconds with that signature move. Rousey also won her three amateur MMA bouts with it.

It isn't the only part of her game, but it's surely what defines her.

Both Continue Improving

Mayweather has lost some (but not much) of his quickness, and over the years has dealt with multiple hand injuries. That hasn't stopped him from aging like fine wine. Against then-undefeated Canelo Alvarez in 2013, Mayweather put forth one of his most dominant performances ever, staying in the pocket vs. Alvarez, a slugger, and weaving in and out while landing right hands from multiple angles.

At times he also came forward and put Alvarez on the defensive. Even though Mayweather's power is more suited to lower weight classes, he delivers punches with enough speed and accuracy to stun opponents. As time has gone on his combinations have decreased, but his single-punch countering has gotten even better. He's also shown the ability to take hard shots, and be more entertaining by engaging opponents when he's on the ropes.

Rousey has also shown other elements to her game besides armbars. Although she proved she can still slap it on high-level opponents, her foes are definitely on the lookout for it inside the octagon. Rousey showed versatility in her two victories prior to Zingano. She disposed of Alexis Davis in 16 seconds in a barrage of punches, showing her striking prowess, and knocked out an elite wrestler in Sara McCann with a knee to the stomach. Just like Mayweather can take an opponent by surprise by moving forward, Rousey can resort to other tactics.

They Can Play The Heel Role

Mayweather became boxing's biggest heel during the buildup to his 2007 bout with Oscar De La Hoya. The "Money" Mayweather persona was born, and a man that boxing fans greatly respected for his skill became a man casual fans either loved or despised for his hubris. While some purists might yearn for the days when he was "Pretty Boy" Floyd, a skillfull master, "Money" Mayweather became one of the richest athletes ever-and without endorsements.

Rousey hasn't become a villain like Mayweather, but she developed a strong base of "haters" after refusing to shake her rival Miesha Tate's hand after their contentious second bout in December 2013. Rousey's never gone to disrespectful lengths like Mayweather, but she has never been shy about telling the world how much she believes in herself, and she's saturated other media with her presence.

Hardcore UFC fans love humble warriors, but Rousey has become arguably the company's most marketable and well-known star for both her skill and exposure.