It’s become the game within the game.

How do you figure out which B.J. Penn is going to show up?

The former UFC lightweight and welterweight champion admitted he sometimes doesn’t know, which is why anytime he steps into the octagon, just about anything can happen.

"It’s definitely a total love and hate relationship with mixed martial arts, and maybe even the UFC at times," Penn said on a media conference call leading up toward Saturday night’s UFC 137 main event against Nick Diaz at the Mandalay Bay Events Center in Las Vegas. "It’s crazy. I want to fight 100 more fights in the next year and the next day I’m like I don’t want to do this anymore.

"I think a lot of fighters feel that way, but maybe I’m just more vocal about it."

The 32-year-old Hilo native will compete in his 21st UFC fight, filling the main event slot for the 12th time in his storied UFC career.

How he got there was an event in itself.

Diaz was originally expected to challenge Georges St-Pierre for the welterweight title before he no-showed multiple press conferences last month.

Penn had signed to fight Carlos Condit in the co-feature bout until UFC President Dana White replaced Diaz with Condit against St-Pierre.

Last week, St-Pierre had to pull out of the fight with an injury, leaving Condit with no opponent.

Instead of finding a replacement fighter to face Condit, White decided to move Penn’s fight against Diaz into the main event slot.

"There’s been so many changes and so many different — It’s been a roller-coaster ride," Penn said. "None of it bothers me.

"It’s always a good thing. I remember the first time I main-evented a card maybe about 10 years ago with Jens Pulver. I just remember being so excited that I was in the main event."

Even more controversy for the fight came about after Diaz’s manager, Cesar Gracie, challenged Penn to a five-round fight, normally reserved for title fights only.

The UFC installed a new rule earlier this year, allowing non-title fights, if they were the main event, to go five rounds. But Saturday’s fight will only go three.

Penn’s last fight could have used an extra two rounds as he fought Jon Fitch to a draw.

He’s 1-0-1 at welterweight since suffering back-to-back loses at lightweight to current champion Frankie Edgar, and a win over Diaz could earn Penn a shot at the winner between St-Pierre and Condit, whenever that fight takes place.

"I don’t want to speak too soon, but I’d love to get another title shot before I step out of the game," Penn said. "But as far as everything goes, I’m just trying to take it one fight at a time.

"If Dana feels that I look impressive in (the Diaz) fight and he wants to grant me that opportunity, I’d be very gracious with the offer."

Penn (16-7-2) is a slight favorite over Diaz (25-7), who has won 10 straight fights since a TKO loss in November 2007 against K.J. Noons, who spent part of his childhood on Hawaii island.

Nine of those 10 wins came by stoppage, as Diaz is regarded as arguably the best boxer in mixed martial arts.

"I really feel that Nick is the best boxer in mixed martial arts today and it shows," Penn said. "In (MMA) fights, he’s got the hardest punch of anyone. He can standup with anyone."

Diaz, who has six weeks to prepare for Penn after missing out on his chance to fight St-Pierre, said he would have rather fought for the title, but thinks Penn provides a bigger challenge in some ways.

"I think B.J.’s a lot better fighter to be honest with you," he said. "(From) a technical standpoint, he’s got much better jiujitsu, boxing .. and I wouldn’t count him out."