LAS VEGAS>> When a state athletic commission official asked Frank Mir where he was between rounds, he named the wrong casino.

That’s a pretty good indication Junior Dos Santos’ punches already were doing their job.

The UFC’s heavyweight champion knew exactly where he was and what he was doing — and Dos Santos swiftly showed Mir the door.

Dos Santos flattened Mir with a huge right hand and finished him on the ground at 3:04 of the second round Saturday night, emphatically defending his belt at UFC 146 on Saturday night.

Dos Santos (15-1) picked apart the two-time ex-champion with superior boxing throughout the fight, eventually sending Mir stumbling onto his back before finishing him with one last blow to the head. Dos Santos then wrapped himself in the Brazilian flag while celebrating his first title defense since taking the belt from Cain Velasquez last fall.

“I’m feeling awesome!” the ever-smiling Dos Santos shouted to the pro-Brazilian crowd. “It’s not bad for a nice guy, huh? … Frank Mir is a really good fighter, too. I came here to defend my belt, and I did it.”

Mir was staggered by multiple blows, including a big body shot, late in the first round, and he barely made it to the bell. After declaring he was at the Mandalay Bay instead of the MGM Grand Garden, Mir was allowed to keep fighting — but the beating didn’t go on much longer.

“My game plan is always to keep the fight on my feet and go for the knockout,” Dos Santos said. “I tried to get him a little tired in the first round, and then go for it. When you believe so much in your performance and yourself, it happens.”

Velasquez stopped Antonio “Bigfoot” Silva late in the first round after administering a bloody beating that both thrilled and horrified fans. Roy Nelson, Stipe Miocic and Stefan Struve also won on a pay-per-view card topped with five heavyweight fights, a first in UFC history.

Dos Santos never faced trouble in the fight’s eight minutes after easily avoiding an opening-minute takedown attempt by Mir, who hoped his superior jiu-jitsu skills would allow him to avoid Dos Santos’ unparalleled striking ability. Mir, who turned 33 on Thursday, has the most heavyweight victories in UFC history, but he couldn’t match Dos Santos’ skills.

“He’s a champ,” Mir said. “He’s fast. I couldn’t get out of the way. He hit me hard. There were just too many of them, and they were hard shots. I couldn’t do anything about it.”

Mir stumbled back several steps after Dos Santos’ decisive right hand. Dos Santos followed him and added one last head shot before referee Herb Dean saved Mir.

“That surprised me a lot,” Dos Santos said. “Man, this guy can take a punch. My hand is hurt.”

Dos Santos downplayed the revenge element of beating Mir, who broke the arm of Dos Santos’ mentor, Antonio Rodrigo Nogueira, in a fight last December.

Mir (16-6) had won three straight fights since losing a title shot to Shane Carwin in March 2010, but couldn’t reclaim the belt he held in 2004 before getting into a serious motorcycle accident and again in early 2009 before losing to Brock Lesnar.

Velasquez (10-1) finished Silva at 3:36 of the first round, but only after pulverizing the 6-foot-4 Brazilian with a relentless series of blows after an early takedown. Silva (16-4) was cut on his face early in the beating, sending streams of blood down his face and onto the canvas, eventually coating both fighters’ torsos.

“I knew he was going to be a tough guy to finish, and he posed certain threats,” Velasquez said. “But I’m happy I was able to go in there and perform. I took my time and waited to get good position to turn it on and finish the fight.”

The fight was stopped once to allow Silva’s corner to clear the blood from his eyes, but Velasquez promptly resumed the beating until the fight was finished. The former Arizona State wrestler was ferociously impressive in his first bout since losing the belt to Dos Santos in just 64 seconds during the UFC’s first prime-time Fox show

“What you’ve done in the past, you’ve got to get over that,” Velasquez said. “This is a step in the right direction.”

The 32-year-old Silva appeared to be outmatched in his UFC debut after a lengthy MMA career highlighted by his dominant Strikeforce victory over Fedor Emelianenko in February 2011.

Nelson (18-7) added another impressive stoppage victory to his list, catching Dave Herman (21-4) with an overhand right that sent him sprawling backward onto the canvas. Nelson landed one more shot before the fight was stopped, and then climbed onto the cage to rub his ample belly while his hometown crowd cheered.

“My plan was to wrestle, (but) my coaches had a different game plan, which was, ‘Hit him in the face,”’ Nelson said. “Guess it worked. Clearly my hands have dynamite in them, or small rocks or whatever.”

Nelson had lost three of his previous four fights, including decisions to Dos Santos and Mir. But the portly, heavily bearded heavyweight — who bills himself as a kung fu fighter — is consistently popular and entertaining.

Miocic (9-0), a firefighter and EMT from Cleveland, remained unbeaten after surviving a back-and-forth first round with fellow heralded prospect Shane Del Rosario. He took control in the second, taking down the previously unbeaten Del Rosario and finishing him off with ground-and-pound elbows.

Del Rosario (11-1) had a severe cut over his right eye and a bloody face after his UFC debut and his first fight in 15 months. The Orange County fighter recovered from a back injury sustained when his car was hit by a drunk driver last year.

Struve finished Lavar Johnson with an armbar just 1:05 into their bout. The 6-foot-11 Dutchman who has lost to Dos Santos and Nelson celebrated his third straight win, while Johnson, the veteran Strikeforce fighter who beat Pat Barry just three weeks earlier, struggled after stepping in as an injury replacement for Australia’s Mark Hunt.

Earlier, veteran lightweight Jamie Varner upset previously unbeaten Edson Barboza, stopping the touted Brazilian prospect with a long series of blows to the head. Varner, the former WEC lightweight champion who was released from that promotion after an 0-3-1 skid, made the most of his chance to be an injury replacement for Evan Dunham in his first UFC fight since March 2007.

Jason “Mayhem” Miller lost a lackluster decision to middleweight C.B. Dollaway, possibly ending the television host’s MMA career. Miller (24-10), who has won just three of his last eight fights, previously said he would retire if he didn’t beat Dollaway.