LAS VEGAS -- For a 118-pounder, Tim Austin hits awfully hard.

Jesus Perez found that out Saturday night when Austin knocked him down four times before referee Kenny Bayless finally moved in to stop the fight at 2:02 of the sixth round.

Austin successfully defended his IBF bantamweight title and remained undefeated in a dominating performance against an unorthodox challenger from Colombia.

The fight was on the undercard of the Evander Holyfield-John Ruiz WBA heavyweight championship fight.

After a slow start, Austin put Perez down in the third round with a straight left hand. He knocked him down again in the fifth round, but Perez got up quickly.

Austin kept up the pressure in the sixth, knocking Perez down early with a left hand. Bayless allowed him to continue, but a few seconds later Austin scored with a right hook that sent the challenger sprawling.

Bayless waved the fight to a close, much to the displeasure of Perez, who argued that he could continue.

It was the 20th knockout for Austin (22-0-1), who weighed a pound beneath the 118-pound class limit. The Cincinnati fighter earned $125,000 for the win.

Perez, also 117 pounds, fell to 20-3. It was his first fight in 21 months.

In a title fight held before only a few early arriving fans, Byron Mitchell regained the WBA 168-pound championship by stopping Manuel Siaca in the 12th round.

Mitchell, 168, of Ozark, Ala., was behind on two cards when he stopped Siaca, 168, of Puerto Rico, at 2:32 of the final round.

Mitchell was a late replacement for Bruno Girard, who vacated his title instead of fighting Siaca because of a purse dispute.

In a women's fight, Christy Martin scored a unanimous 10-round decision over Jeanne Martinez that was booed by the crowd.

Martinez, 145, of Slidell, La., didn't give any ground but was outboxed by Martin, 144½, who won by four points on one card, six on a second and eight on a third.

Martin, of Orlando, Fla., improved to 42-2-2, while Martinez fell to 13-5-2.