Unheralded Russian middleweight Dmitry Pirog stopped Daniel Jacobs with a crushing overhand right in the fifth round to win the vacant WBO title Saturday night.

Former featherweight champion Robert Guerrero dominated Joel Casamayor in a lopsided unanimous decision to win his debut at 140 pounds, and Jorge Linares also earned a comfortable unanimous decision over Rocky Juarez on the stacked undercard of Juan Manuel Marquez's lightweight title rematch with Juan Diaz at the Mandalay Bay Events Center.

The lively crowd witnessed a landmark win for the 30-year-old Pirog (17-0, 14 KOs), a star amateur in Russia before a solid pro career. Pirog fights with a distinctly Western style, and his Vegas debut fulfilled his lifelong goal to fight in boxing's capital city.

The Russian was a smash hit in Sin City: After four lively rounds, he stopped the favored Jacobs with one punch in the fifth, putting the previously unbeaten Brooklyn prospect flat on his back.

Pirog briefly stood over Jacobs with his arms raised before celebrating his first world title.

"I'm very happy and thankful I'm taking this belt back to my fans in Russia," Pirog said. "Danny punched very quickly and sharply, but I didn't feel much from him. I hurt him in the second round, and I knew I could come back and do it again."

Although referee Robert Byrd barely got into his count before waving off the fight, Jacobs believed he could have kept going.

"Everything happened so fast," Jacobs said. "I got caught with a shot I didn't even see. ... I'm not going to complain. I just hope everyone will forgive me and keep the faith. I know I can come back and become a champ. I think if I was at 100 percent, I would have done a little better. It's just one of those things that happens."

Jacobs fought one week after the death of his beloved grandmother, Cordelia Jacobs. He wore her nickname, "Lady Bird," on the front of his trunks, and planned to catch a quick flight back to New York for her funeral Sunday.

Earlier, Guerrero knocked down Casamayor in the second round and barely let up afterward, cruising to his fourth straight victory with superior speed and power.

"I felt good, and I had a dominant performance," Guerrero said. "He's a lefty, and he's really crafty with a good counterpunch. He kept using his head and grabbing me to hold me down. He's a veteran, and he knows how to survive."

The 39-year-old Casamayor (37-5-1) managed to knock down the Bay Area fighter late in the final round with a sneaky right hook, but Guerrero appeared to be more surprised than hurt after his first taste of the canvas in his boxing career.

Two judges and The Associated Press scored it 98-89, and a third had it 97-90.

Guerrero (27-1-1, 18 KOs) is fighting with a clearer head in recent months with the improved health of his wife, Casey, whose leukemia is in remission after he surrendered his super featherweight belt to care for her.

The 27-year-old Guerrero's ascension has been delayed by his wife's disease and squabbles with his promoters, but the former featherweight champion appears primed to make an impact in the crowded 140-pound division if he stays there. Guerrero said he would prefer to fight the winner of Saturday night's main event, even going back down to 135 pounds to do it.

Casamayor, the Cuban Olympian-turned-defector who became a champion at 130 and 135 pounds, quickly realized he couldn't match Guerrero's speed. After referee Jay Nady penalized Casamayor for blatant holding in the second round, Guerrero rocked Casamayor late with a left hand and knocked him down moments later, leading to a 10-7 round.

"In the first two or three rounds I was a little confused, but I got my rhythm back," Casamayor said, dismissing any thought of retirement. "Robert is tall, so it was a little awkward. I was a little rusty, and I got my rhythm too late. I'll still fight anyone."

Linares (29-1, 18 KOs) largely dominated his lightweight fight with Juarez (28-7-1), the longtime title contender and U.S. Olympic silver medalist who has won just once in his last six fights. Juarez said he'll contemplate retirement.

Earlier, Sakio Bika was disqualified after knocking Jean-Paul Mendy unconscious with an uppercut while Mendy's knee appeared to be down on one knee.

Mendy (29-0-1) remained facedown on the canvas for several moments, eventually going to a hospital. Manager Gabriel Gaide said Mendy would be fine, and looked forward to fighting Lucian Bute for the IBF 168-pound title.