LAS VEGAS – B.J. Penn and Lyoto Machida headline different shows for the UFC this weekend. Those fights will take place four weight classes apart, which makes it all the more difficult to comprehend that the two fighters once faced off in a heavyweight bout.

Penn (16-9-2 MMA, 12-8-2 UFC) meets Frankie Edgar (16-4-1 MMA, 10-4-1 UFC) in the featherweight main event of the TUF 19 Finale on Sunday, while Machida (21-4 MMA, 13-4 UFC) challenges Chris Weidman (11-0 MMA, 7-0 UFC) for the middleweight world championship just 24 hours earlier tonight at UFC 175.

Both events take place at Las Vegas’ Mandalay Bay Events Center with the TUF 19 Finale airing on FOX Sports 1 and UFC 175 available on pay-per-view.

When Penn and Machida went to battle more than nine years ago under the K-1 banner, it marked the only time “The Prodigy” has ever been defeated outside of the UFC.

With the matchup just the sixth professional fight of Machida’s career, few knew what “The Dragon” was capable of and what he would go on to accomplish in his career. Penn admitted he also didn’t know much about Machida at the time of the fight, but when the challenge was put forth, he couldn’t help but accept.

“From my understanding, the fight got put together because K-1 called us and said Machida and them have a guy that wants to fight you,” Penn told MMAjunkie. “(They said), ‘He’s a really good street fighter, but no one knows him. I said, ‘No one knows him? Tell Machida I’ll fight him.’ Then it got put together.”

Penn, who has fought the majority of his career at lightweight and welterweight, ballooned up to more than 200 pounds for the March 2005 bout. Even with the added size, he was still at a massive disadvantage. He was the significantly smaller fighter and lost the speed and power advantages he enjoyed when competing at a natural weight.

Because of that, he was forced to make compromises with his fighting style, the most meaningful of which was conserving his energy for a one-shot knockout blow.

“I remember I had a game plan where I was going to go swing and try to knock him out when he grabs me and pushed me on the ropes,” Penn said. “I wasn’t going to use as much energy so I could get my chance to knock him out again.”

Penn failed to execute that game plan and went on to lose a unanimous decision to his Brazilian opponent. However, he delivered a respectable effort in the process.

Even though he’s had numerous fights since then, Penn fondly remembers his bout with Machida, particularly because he was forced to absorb one of the most powerful body shot strikes of his career.

“I remember him kicking me so hard in my ribs one time – it was crazy.” Penn said. “That’s how the fight went and he ended up walking away the victor.”

Despite the fact he didn’t defeat Machida, the Hawaiian has had an extremely successful career that’s included two UFC championship reigns in separate weight classes. Machida hasn’t achieved quite that level of overall success, but he’ll have the chance to put his name right there with Penn in the record books when the former light heavyweight champion attempts to take middleweight gold from Weidman tonight.

For more on UFC 175 and The Ultimate Fighter 19 Finale, stay tuned to the UFC Rumors section of the site.