LAS VEGAS (Ticker) -- WBA heavyweight champ John Ruiz has been given little respect despite holding a share of one of boxing's top prizes. On Saturday, he fights for more when he defends his title against highly regarded contender Kirk Johnson.

Despite coming into the fight as the champion, Ruiz (37-4-1, 27 KOs) is seen as a decisive underdog by many against the unbeaten challenger.

"I am ready to fight," Ruiz said. "Being an underdog does not bother me. It fires me up."

Ruiz claimed the belt that was stripped from Lennox Lewis in April 2000. Lewis had unified the heavyweight crown five months earlier when he outpointed Evander Holyfield, but Lewis opted to fight a high profile opponent in then-undefeated Michael Grant rather than Ruiz.

The unheralded Ruiz met Holyfield for the vacant title, but lost a questionable decision in August 2000. In the rematch, Ruiz was vindicated as he dropped Holyfield in the 11th round en route to a unanimous decision in March 2001 to become the first Latino heavyweight champ.

Ruiz and Holyfield battled for a third time on December 15 and Ruiz was lucky to escape with his title as they fought to a draw in Mashantucket, Connecticut. Holyfield kept himself in line for another title shot as he recorded an eight-round technical decision over Hasim Rahman on June 1.

"Most people say it was three great fights with Evander," Ruiz said. "I am just looking forward to moving on in my career and this is my opportunity to do that. This fight with Kirk Johnson is going to be a tough one, but I am more than ready."

The overachieving Ruiz, who has limited tools but gave Holyfield trouble with an unorthodox style, should not present the same problems for Johnson.

A heralded amateur from Canada, Johnson (32-0-1, 23 KOs) has been waiting patiently for a title shot, while making a steady climb up the heavyweight ranks since turning pro following the 1992 Olympics. A boxer-puncher, Johnson earned a shot at the title with a steady, but unspectacular performance as he outpointed Larry Donald in a WBA title eliminator last summer in his last fight.

"This is a lifetime dream, and I am finally living my dream," Johnson said. "It is a great opportunity for me to showcase my skills and my talent."

Also on the undercard, a pair of titles will also be on the line.

Unbeaten IBF bantamweight champion Tim Austin (24-0-1, 21 KOs) will be looking to make the ninth successful defense of his 118-pound title when he faces Adam Vargas (37-3-1, 23 KOs) while WBA 168-pound super middleweight champ Byron Mitchell (24-1-1, 17 KOs) meets former middleweight champ Julio Cesar Green (25-4-1, 17 KOs) in a non-televised title fight.

The bouts will be televised on HBO beginning at 10:00 p.m. EDT.