LAS VEGAS -- The more blows Jose Armando Santa Cruz landed, the more the damage on David Diaz's face became apparent.
A cut in Diaz's hairline.
A big welt and bruise around his left eye.
More swelling around his right eye.
But Diaz never stopped trying, never stopped coming forward and trying to win the fight.
Although he was trailing badly, Diaz scored two knockdowns in the 10th round before referee Richard Steele called it off at 2:26 Saturday night to win an interim lightweight title in upset fashion.
The fight was one of the featured bouts on the undercard of the Hasim Rahman-Oleg Maskaev heavyweight championship bout at the Thomas & Mack Center.
It was a stunning comeback for Diaz (32-1-1, 17 KOs), who trailed by five points on two scorecards and three points on the third.
"I told you I would steal the show," said Diaz, a 1996 U.S. Olympian in the biggest fight of his career. "I studied two of his videotapes and I discovered he can't take it to the body. I won this for Chicago and I promised my mother [Basilisa, who has recovered from a kidney transplant] I would win the fight."
Santa Cruz (23-2) had busted up Diaz's face and was landing the heavier shots, and more of them. He landed 284 of 936 blows (30 percent) while Diaz connected on just 129 of 566 (23 percent).
But a few of Diaz's shots did damage in the 10th round. He landed a left hand that stunned Santa Cruz and another that dropped him against the ropes.
Diaz, 31, knocked him down again a few moments later, and Santa Cruz, 25, was wobbly when he got up. Diaz attacked again with a flurry, forcing Steele to step in.
Diaz, who took a two-year sabbatical from boxing before coming back four years ago, certainly had the size advantage in the fight. Both made the lightweight limit of 135 pounds -- they were both officially 134½. However, Diaz was 150 when he arrived at the arena on HBO's unofficial scale while Santa Cruz had rehydrated to only 142 pounds.
Although the victory netted Diaz an interim belt, the more important element of his win is that he became the mandatory challenger for the division's recognized champion, Diego Corrales.
Soto dominates Valle
Humberto Soto (40-5-2, 24 KOs) knocked out his Los Mochis, Mexico, crosstown rival Ivan Valle (24-8-1) in the fourth round of a foul-filled bout to become the mandatory challenger for junior lightweight champion Marco Antonio Barrera.
Soto landed two left hands for a knockdown in the first round, and that was the start of the craziness.
Valle opened a cut over Soto's left eye in the second round and then drilled him in the groin, sending Soto to the canvas in agony.
Referee Joe Cortez docked Valle two points for the flagrant foul.
In the third round, Soto scored two more knockdowns and was warned by Cortez for delivering his own low blow.
Soto, the cut getting worse, ended it at 42 seconds of the fourth when he knocked Soto down for the fourth time with a flush right uppercut.
• Junior middleweight Vanes Martirosyan (10-0) easily outpointed game Marcus Brooks (6-2), who absorbed a lot punishment in a spirited six-rounder.
Martirosyan, a 2004 U.S. Olympian, won 60-53 on two scorecards and 60-52 on the third. Just 20, Martirosyan was getting his first television exposure in the opening fight of the HBO PPV card and looks like a prospect to watch.
He thrilled the hundreds of vocal fans who made the trek from Glendale, Calif., to support him by battering Brooks all over the ring for most of the first round. He smacked Brooks with numerous right hands, and finally knocked him down for a seven count just moments before the round ended.
But Martirosyan couldn't finish Brooks, who rallied a bit in the fourth by clipping him with some stinging rights. Martirosyan said the reason he didn't score a knockout after such a fast start was because he injured his right hand in the first or second round, no surprise considering how many rights he landed. Martirosyan went to the hospital afterward to have the hand examined.
• Super middleweight Aaron Pryor Jr. (6-0), the son of Hall of Famer Aaron Pryor, squeaked out a split decision victory over Danny Jevic (7-8-2) in a six-round super middleweight bout. Two judges had it 57-56 for Pryor while the third had it by the same score for Jevic.
Pryor, clearly not going to the top like his father once did, struggled mightily over the last two rounds, seemingly exhausted to the point where trainer Emanuel Steward threatened to stop the fight if he didn't pick up his energy level.
Dan Rafael is the boxing writer for ESPN.com