VIDEO Highlights Andy Ruiz vs. Anthony Joshua: Breaking Down One Of Heavyweight Boxing's Biggest Upsets

In arguably the biggest underdog story of the sports year, Andy Ruiz Jr. has found himself on top of the boxing world with his shocking TKO victory Saturday over Anthony Joshua at Madison Square Garden.

It was understandable for Ruiz to enter the fight as an 11/1 underdog. While the undefeated Joshua looked like his typical chiseled self, Ruiz's round physique didn't inspire too much confidence that he could defeat the unified WBA/WBO/IBF/IBO champion. Joshua also had an eight-inch reach advantage to go along with a four-inch height advantage.

Ruiz, no doubt, knew what he was up against. Joshua is a fundamentally sound boxer and often outclasses his opponents by dealing a steady number of jabs and well-timed power punches. For much of the fight, Joshua showed why he's among the best heavyweights of his generation and one of the best pound-for-pound boxers alive.

But there was plenty of fight in Ruiz and the victory was no fluke.

The 29-year-old Southern California native overcame a fierce left-hook knockdown in the third round, and while Joshua was trying to finish him off, rallied to drop the Brit twice in the round. Joshua seemed to regain his composure, winning the next two rounds.

But Ruiz appeared to win the sixth and then knocked down Joshua twice in the seventh before the fight was called off by referee Michael Griffin.

So how exactly did such an upset happen?

Joshua Has Fatigue Problems

Much like when he was knocked down in April 2017 by Wladimir Klitschko, Joshua showed that he is capable of punching himself out of fights. Though he defeated Klitschko, Joshua looked exhausted during stretches and Klitschko somehow kept the fight competitive.

After knocking down Ruiz in the third round, Joshua again seemed to run out of steam. It probably would have been a better strategy to bounce around the ring and have Ruiz chase him after the knockdown but Joshua felt compelled to try to finish the fight a little too soon, leaving himself vulnerable to counterpunches that ultimately doomed him.

In the final minute of the sixth round, Joshua barely threw any punches. By the seventh, he barely could fight back against Ruiz's barrage of punches.

Ruiz Can Take A Punch

The knockdown in the third round would have kept most boxers on the mat. However, Ruiz proved that when he's stunned he is capable of battling back. It was an impressive display of courage from such an underdog to keep going and it paid off.

Many Overlooked Ruiz's Past Record

It may come as somewhat of a surprise that Ruiz has actually fought a decent number of established opponents. After losing a majority decision to highly touted Joseph Parker and veteran Kevin Johnson, he probably understood what he was up against when he faced a superstar like Joshua.

He entered the fight with a 32-1 record with 21 knockouts.

No Hometown Advantage For Joshua

It's hard to gauge how important it is to fight on your home soil, but this was Joshua's U.S. debut and it didn't go so well. He has mostly fought at London's Wembley Stadium and O2 Arena, where his opponents may have felt a little uncomfortable.

Also, he may simply have not been prepared for this opponent. Joshua was originally scheduled to fight undefeated New Yorker Jarrell "Big Baby" Miller, who was dropped after failing doping tests.