LAS VEGAS (Ticker) -- Kostya Tszyu unified the WBC and WBA super lightweight titles Saturday night when Sharmba Mitchell was unable to continue at the end of the seventh round after he aggravated a knee injury.

Tszyu (ZOO) won an ugly bout and is slated to meet IBF champion Zab Judah to unify the 140-pound title on May 19.

Before the fight, rumors circulated that Mitchell's left knee was not 100 percent. But he said that he was ready and "going in like a champ."

"I trained too hard to quit in the dressing room," Mitchell said. "I trained too hard to quit on this stool. It was my corner's decision."

The fight was closely contested, but it was Tszyu (26-1-1, 22 KOs) stepping up the attack as the bout progressed.

Less than a minute into the seventh, Mitchell appeared to aggravate his old injury, one which required surgery several years ago.

While his corner was feverishly massaged his knee between the seventh and eight rounds, Mitchell was visited by the ringside doctor. With Mitchell complaining, "I can't move, it's numb," the fight was stopped by referee Joe Cortez.

"I told my corner that I wasn't really feeling my leg, but I didn't want to stop it," said Mitchell, who fell to 47-3. "They really didn't want me to fight, but I wanted to fight -- that's just me.

"Apparently he had problems with his leg, and I believe his corner stopped it," Cortez said. "He was complaining about his leg, that is was kind of hurting. We have to watch out for the safety of the fighters. When a fighter is in the ring, especially when he is not 100 percent, a fighter can get hurt with a fighter like Kostya Tszyu."

At the time of the stoppage, judges Chuck Giampa and Daniel van de Wiele had Tszyu ahead, 68-65 and 68-64. Judge Keith MaDonald had the fight even, 66-66.

Mitchell showed no effects of the injury in the opening round. The flashy southpaw utilized his speed and movement, working the perimeter of the ring while sticking in his right jab.

"He knows what boxing is all about," Tszyu said. "He said he was sick before the fight, but I really didn't notice it. He came into the ring. No excuse. It could have been a trick. Who knows? Sharmba is a smart man."

Midway through the round, Tszyu stepped on Mitchell's foot while landing a right hand, sending him to the canvas. It was not ruled a knockdown.

By the second round, Mitchell had blood pouring from his nose due to Tszyu's roughhousing, straight-forward pressure. But Mitchell finished strong, landing successive right-left combinations to the head. In the fourth, it appeared Mitchell hurt his leg in the corner as the action began to heat up. He hurt Tszyu midway through the round before Tszyu had a point deducted by Cortez for throwing Mitchell to the canvas.

"Sharmba is a good fighter," said Tszyu. "Some of my punches I threw were pretty sharp, and he defended. He has good defense."

"I am a fighter, I am a warrior, I am a champion," Mitchell said. "I came to fight, and he couldn't beat me with a hurt leg. Get me when I am 100 percent, not 50 percent. It was unfortunate my leg went out on me."