They entered the Octagon with a ton of respect for each other, but when the horn sounded to begin fighting, that respect was placed on hold.
But Saturday night at UFC 137 in Las Vegas, Diaz showed no respect for the former UFC champion. He used his 6-inch reach advantage to land a hard overhand right. That was just the beginning. Diaz would go on to beat up Penn en route to a unanimous decision victory.
The fight was scored 29-28, 29-27 and 29-28. ESPN.com had it 29-28 for Diaz.
"B.J. is the man," Diaz said. "I always loved watching him fight."
After the hard-fought battle, Penn's face was a puffy mess: His left eye was swollen shut, and bruises were splattered across his head. It was enough to make him say he wouldn't fight anymore.
"This probably the last time you will ever see me in here," Penn said after falling to 16-8-2. "I've got a daughter at home, another on the way. I don't want to go home looking like this; I'm done."
If this is the end for Penn, he went out on his shield.
Early in the fight, Penn was able to get under Diaz's long reach and land punches to his body. Penn even took Diaz (26-7, 1 no-contest) to the ground a good opening round for the former lightweight and welterweight champion.
But in the second round, the bigger and stronger Diaz began wearing Penn down. He landed several hard punches to the body, and a left to the chin dazed Penn. Diaz would increase the assault on Penn's face. With a little more than a minute remaining in the second, blood began flowing from Penn's nose. Diaz was proving to be too big and too strong.
In the third, Diaz began pouring it on. He pushed Penn against the cage and punish his body. Then Diaz stepped back and landed right-left combinations.
Penn's left eye began to swell, but he refused to go down without a fight. He landed right hands that drew blood from Diaz's left eye.
But when the battle was over, there was no doubt that Diaz had won this war. And he wasted no time calling out welterweight champion Georges St. Pierre. During the postfight news conference, UFC president Dana White granted Diaz his wish.
Diaz, the former Strikeforce welterweight champion, was originally scheduled to fight St. Pierre in the main event of this card. But he repeatedly missed prefight obligations and was removed from the title bout.
"I don't think Georges is hurt," Diaz said. "I think he's afraid to fight anybody."
Carlos Condit, who was slated to face Penn, replaced Diaz. But St. Pierre later injured a knee and withdrew from the card.
Kongo hands Mitrione his first pro loss
Cheick Kongo and Matt Mitrione spent most of the first two rounds of their heavyweight fight being cautious. In the third, however, Kongo took Mitrione to the ground and unleashed on him. The late aggression secured a unanimous decision for Kongo.
The judges scored the fight 30-27, 30-28 and 29-28.
Kongo and Mitrione were overly respectful of the other man's punching power. Very few strikes were delivered in the first round.
Kongo came out more aggressive in the second round, landing several kicks to Mitrione's right shin. But Kongo's offensive output eventually slowed.
He would slam Mitrione to the ground in the third round and deliver many hard punches. Mitrione suffered a cut over his left eye courtesy of one of Kongo's flurries.
The loss was Mitrione's first as a professional mixed martial artist. He falls to 5-1.
Nelson finishes Cro Cop in third round
"It was awesome to compete with a legend," Nelson said of Cro Cop.
Nelson, who was criticized by UFC president Dana White for not being in good physical condition in May after a loss to Frank Mir, showed improved cardio against Cro Cop.
He was able to get the fight to the ground and absorbed several hard strikes. Nelson (16-6) overcame a hard right hand and several kicks to his midsection in the second round to take Cro Cop down. Once there, he applied a crucifix and nearly pounded out Cro Cop, who was saved by the horn.
But in the third, Nelson would get the back of Cro-Cop. While on the ground, Nelson landed punch after punch.
Cro Cop was no longer able to defend himself and the fight was stopped. It was likely the last fight of his career.
"I said before this fight, I said it would be my final fight," Cro Cop said. "I want to thank you all, especially UFC fans."
Filipovic fell to 27-10-2, with 1 no-contest. He has lost three fights in a row.
Jorgensen grounds out a win over Curran
Bantamweight contender Scott Jorgensen welcomed Jeff Curran back into the Octagon by putting on a wrestling display.
Jorgensen took Curran to the ground several times in the bout to earn a unanimous decision. The judges scored the fight 30-27, 29-28 and 29-28.
ESPN.com also scored the bout in favor of Jorgensen, 30-27.
Jorgensen (13-4) added a wrinkle to his defensive game -- jiu-jitsu. Curran never could get a solid submission attempt on Jorgensen, who showed improved defensive skills on the ground.
"I wanted to play on the ground," Jorgensen said. "It's the first time I've ever had a jiu-jitsu coach."
Curran, who returned to UFC action for the first since January 2004, slips 33-14-1.
Hioki edges Roop to win UFC debut
Like so many Japanese mixed martial arts stars, Hatsu Hioki found the Octagon a difficult place to adjust. Still, Hioki was able to walk away victorious in his UFC debut by defeating George Roop via split decision in a featherweight bout.
Two judges scored the fight 29-28 for Hioki; the third had it for Roop 29-28. ESPN.com scored the fight 29-28 for Roop.
"He was tough," said Hioki, who is ranked third among featherweights by ESPN.com. "He was stronger than I expected."
Roop was the more aggressive fighter standing, landing several kicks and hard right hands. But Hioki was able to control much of the action on the ground.
Hioki's best round was the third. He was able to get Roop to the ground and keep him there.
But while on the ground, Hioki (25-4-2) did little damage. Roop avoided any submission attempts by Hioki and was rarely hit.
The loss dropped Roop to 12-8-1.
Cerrone continues impressive run, submits Siver
Cerrone applied a rear-naked choke that forced Siver to tap at the 2:22 mark.
"I'm just here to keep fighting," said Cerrone, who won his sixth fight in a row. "I want to get another fight in December. If I can do that, that would be great."
Cerrone (17-3, 1 no-contest) did most of his damage in the stand-up. He wobbled Siver early with a hard left kick to the chin.
Siver (19-8) survived that onslaught but not the striking assault that followed soon after.
When Cerrone had Siver hurt again, he went for the submission and got it.
Palaszewski knocks out Griffin in first
Once-highly touted lightweight Tyson Griffin failed to make weight for his featherweight fight, then couldn't make it out of the first round.
Bart Palaszewski, making his UFC debut after previously competing in WEC, landed a hard left hook and a right that sent Griffin to the canvas. Griffin got to his feet quickly but was still dazed.
That's when Palaszewski jumped on him, landing several left-right combinations that knocked Griffin out at 2:45.
"I've been preparing," said Palaszewski, who improved to 36-14. "We've had guys his size, wrestlers, coming in. I just picked my spots."
Griffin (15-6) was hoping to fight for the second straight time at featherweight but could not make the required 146-pound limit for the nontitle bout. He tipped the scale at 149 pounds.
The bout was fought at a catchweight of 148 pounds.
Vera survives late rally by Marshall
All three judges and ESPN.com scored the fight 29-28 for Vera, who improved to 12-5 (1 no-contest).
It was a huge win for Vera, who was released by UFC after suffering a loss to Thiago Silva in January. That loss was overturned and deemed a no-contest when Silva failed a postfight drug test.
Marshall slipped to 10-4 after dropping his second fight in a row.
After losing the first two rounds, Marshall nearly pulled out the win with an impressive third round. He stunned Vera with a right hand and with seconds remaining in the fight nearly submitted him with an armbar.
Nijem overpowers Downes
Ramsey Nijem proved too fast and too strong for Danny Downes in their lightweight bout. Nijem dominated for much of the fight to come away with a lopsided unanimous decision.
The judges scored the fight 30-25, 30-26 and 30-27. ESPN.com scored it 30-27 for Nijem.
Downes gave Nijem a scare in the second round when he nearly pulled off a submission. But Nijem, who returned to lightweight after competing in the welterweight division in his previous UFC bouts, escaped.
Nijem improves to 5-2 overall and 2-0 at 155 pounds. Downes (9-3) suffers his second loss in a row.
Carmont shuts out Camozzi
The judges scored the fight 30-36, 30-27 and 30-27 for Carmont, who made his UFC debut. ESPN.com also had Carmont as the winner, 30-27.
Carmont (17-7) was in control throughout the fight standing and on the ground. He landed several knees and connected with a right-left combination in the second that floored Camozzi.
Camozzi fell to 15-5.
Starks beats Jacoby in UFC debut
Middleweight Clifford Starks was slightly more aggressive in the stand-up and was able to take Dustin Jacoby down in each round to earn a unanimous decision in his UFC debut.
All three judges scored the fight 30-27, as did ESPN.com. Starks improved to 8-0.
Jacoby, who also made his Octagon debut, suffered the first loss of his pro career. He is now 6-1.
Franklin McNeil covers mixed martial arts and boxing for ESPN.com. Follow him on Twitter at www.twitter.com/Franklin_McNeil.