LAS VEGAS (Ticker) -- John Ruiz retained his WBA heavyweight title Saturday night when challenger Kirk Johnson was disqualified late in the 10th round by referee Joe Cortez for repeated low blows.

Ruiz (38-4-1) was ahead on all three judges' scorecards when the fight was halted.

"I was fighting clean all the time," Ruiz said. "He tried to hit me low, throw me off my game plan. I still want to have kids, I don't know what was going on in his head."

Johnson (32-1-1) had points deducted in the first round and was issued a "stern warning" by Cortez in fourth. He had another point deducted in the seventh and was disqualified with 43 seconds left in the 10th.

"He was always crouching under, a couple of times I strayed low," Johnson said. "It is my fault, I should have not thrown that last blow. It was borderline."

A serious clash in styles resulted in an uneventful fight. Ruiz's awkward, bruising style did not mix with Johnson's more technical approach and the result was a sloppy bout with little action, lots of holding and occasional wrestling.

The constant infractions wore on Ruiz, who head-butted Johnson late in the fourth round. Cortez threatened both fighters that they were risking disqualification, but the dirty fighting continued.

Unintentionally, Johnson landed a left hook below the belt in the seventh as Ruiz crashed to the canvas, wincing in pain.

In the ninth, the champion stunned Johnson with a counter right to the face. Johnson tried to grab Ruiz as both fighters tumbled to the canvas.

Sensing the tide turning, Ruiz came out quickly in the 10th. In the final minute, however, Johnson landed a left to the groin that prompted Cortez to disqualify the challenger.

"He hit me low throughout the fight," Ruiz said. "I want to take on that girl, Lennox Lewis, to show who the real world champ is."

On the undercard, unbeaten IBF bantamweight champion Tim Austin retained his title with an impressive 10th-round stoppage of Mexico's Adan Vargas.

Making his ninth title defense, Austin (25-0-1, 22 KOs) was given a stiff test by Vargas in a highly competitive fight.

But Austin's power finally wore down the challenger as the champion began landing heavy punches in the 10th round. With 63 seconds left, referee Tony Weeks stopped the bout.

Vargas, who failed in his third try at a bantamweight world title, fell to 37-4-1.

Also, WBA super middleweight champ Byron Mitchell (25-1-1, 18 KOs) was lucky to escape with his title as he was dropped twice in the opening round by challenger Julio Cesar Green (25-5-1). Mitchell retained the belt when the fight was stopped by the ringside doctor in the fourth due to a cut over the challenger's left eye.