Sugar Ray Leonard knocked down Adolfo Viruet in the fourth round and went on to a welterweight fight at the Dunes Hotel.

It was the 21st professional win against no losses for the former Olympic gold medalist from Palmer Park, Md., who weighed in at 144. Viruet, 147 1/4, from New Jersey, fell to 17-4.

One of Viruet's other losses came at the hands of former lightweight champion Roberto Duran. It was a 10-round decision. Viruet, 27, is a former two-time New York State Golden Gloves champion.

Leonard, 22, the No. 6-ranked welterweight challenger on the World Boxing Association lists and No. 10 in the World Boxing Council standings, floored Viruet with a sharp right in the fourth, the first time Viruet had been knocked down in his professional career.

Judge Hal Miller scored it 48-42, judge Art Lurie 47-44 and Duane Ford 47-42.

Leonard scored with left jabs early in the fight, building up a commanding lead. He appeared to coast during the middle rounds and then changed his tactics against Viruet, an awkward southpaw. Viruet won only one round, the ninth, on all three cards.

It was the first time in Leonard's professional career that he had fought a left-hander, and he admitted the adjustment was tough.

"He was very awkward, crafty and tricky," Leonard said. "He had the experience that when he was hurt he was able to shake it off. I wasn't looking for the knockout except for the fourth round when I had him in trouble. But when he didn't go out in the round, I knew it was going to go the distance.

"He never really hurt me. He hit me with a couple of awkward shots, if you want to call being hit in the back of the head awkward. He wasn't the toughest fighter I have faced, but he was the most determined to beat me."

Leonard earned $200,000 for the victory. Viruet received $40,000.

Leonard will fight May 20 against Marcus Geraldo in New Orleans.

After the fight, Viruet said he might have broken his right hand in the foruth round.He said he will have the hand X-rayed.

Leonard had fought as a professional in Las Vegas just once before. He knocked out Augustine Estrada of Mexico at 1:54 of the sixth round at Caesar's Palace on Nov. 5, 1977, for his fifth straight pro victory.