PHOENIX -- Former IBF cruiserweight champion Vassiliy Jirov battered Joseph Kiwanuka into submission in six rounds Thursday night to win the North American Boxing Organization cruiserweight belt and improve his chances to challenge for a major title again.

Jirov (33-1, 29 KOs) punished Kiwanuka (27-6-2) from the start.

The Ugandan, now living in Las Vegas, tried to answer the bell for the seventh round of the scheduled 12-rounder, but ringside physician Robin McDougal examined a cut over Kiwanuka's right eye and nodded to referee Bobby Ferrara. He stepped in and signaled that the bout was over.

It was the second straight fight Kiwanuka wasn't able to continue. His last was stopped after seven rounds against Julio Gonzalez on April 27, 2002, after he sustained cuts over both eyes and a broken nose.

Jirov, who won a 1996 Olympic gold medal for Kazakhstan but now lives in suburban Fountain Hills, Ariz., also has been relatively inactive.

The bout was only his third in nearly two years and the second since James Toney outpointed him in 12 rounds April 26 for the IBF crown.

Jirov knocked out Ernest Mateen in the seventh round Aug. 7, and in this fight used his superior weight and strength for his easiest victory on the comeback trail.

The left-handed slugger weighed in at 199 pounds -- 9½ more than Kiwanuka for a fight in which the NABO set the limit at 200 -- and systematically wore out the smaller man.

Jirov used his jab effectively the first two rounds, then began an assault on Kiwanuka's body. By the fifth round, Ferrara had issued two low-blow warnings to Jirov, but Kiwanuka was holding his right elbow close to his side in an attempt to stop the punishment from Jirov's left hook.

In the sixth, Jirov threw 30 straight jabs, landing about half, to set up a crushing left to the head.

On the undercard, two well regarded fighters were upset in split decisions.

Cristian Favela (19-5-3) of Mexico outpointed former WBC super bantamweight champion Willie Jorrin (29-2-1) of Sacramento in a 10-round co-main, and journeyman Willie Chapman of Ogden, Utah, gave Malcolm Tann of San Antonio, a former Golden Gloves heavyweight champion, his first loss.

Chapman (18-18-3) knocked Tann (8-1) down with a right cross with 24 seconds left in the second round and came out ahead on two of three scorecards.