Thomas Hearns threw just one good punch tonight. But that was all he needed to retain his World Boxing Association welterweight title tonight.

Hearns staggared challenger Pablo Baez with a right hand midway through the fourth round, then chased him into the ropes and pounded him for 30 seconds before referee Ken Morita stopped the fight 2:10 into the round and awarded Hearns a technical knockout.

The easy victory was the 32nd consecutive win for Hearns as a professional, his 30th by a knockout. The 22-year-old champion controlled the fight through three rounds, setting up his right with a series of left hooks before finishing Baez with a right hand in the fourth.

"The right hand came right after a left hook that left him open," Hearns said minutes after leaving the Astrodome ring to moderate applause. "The second and third rounds I was using my jab to set up the right hand. The right hand opened him up for me and I was able to hit him with some lefts and then some rights before he (Morita) stopped it."

The fight we expected to be a warmup for Hearns' Sept. 16 bout with Sugar Ray Leonard. It was. Hearns had tried all week to make Baez sound like a legitimate opponent. But Baez, 23, who received his No. 9 ranking by the WBA only after sizing for this fight, was incapable of hurting Hearns.

"No, he didn't hurt me," Hearns said. "But he took my punches well. After I hit him a few times in the first round and didn't get to him, I had to regroup. That is when I started using my left hook to try to set myself for the right."

Hearns seemed to toy with Baez for much of the fight, even laughing at him after the challenger threw a wild left-right combination that didn't even come close.

In the fourth, Hearns moved in slowly, threw his left hook then stood up Baez with the right. As Baez fell back toward the ropes, Hearns closed in with his left.

When Baez's hands dropped, Hearns finished him off with a series of rights, finally forcing Baez to slump to the bottom ropes as Morita moved in to prevent further damage.

Although he spent most of the postfight press conference building up Baez, Hearns could not resist a few early shots at Leonard. "I don't think he will be able to take my punches as well as Baez did. That fight for me tonight was a lot harder than it looked. He took some good punches.

"When I fight Leonard, he will pull the same stunt Duran did. He will quit. Only this time you'll know the reason why. But I don't want to talk about that yet. I want to talk about Baez."

Unfortunately for Hearns, there was not much to say about Baez. Neither fighter seemed to inspire the imagination of the crowd at any point. Baez never amounted an attack and Hearn's punches were perfunctory until round four. And, given the intensity of the Leonard-Ayub Kalule fight that followed, this was not a particularly good night for Hearns, even though he collected $500,000 for his third defense of the title he won Aug.2 on a second-round knockout of Jose (Pipino) Cuevas. Baez earned $75,000.

"I did have the fight under control the whole way," Hearns admitted. "I spent most of the time just trying to set him up for the one good punch. But it wasn't as easy as people will make it out to be."

As for the less-than-enthusiastic reception he received, Hearns said, "I think I am accepted as the champion, but I have one problem -- there is someone else in my weight class (Leonard) who is also the champion. But, now I can concentrate on eliminating that problem." Hearns will need more than one good punch to rid himself of Leonard. Clearly the subject of his coachampion upsets him. When someone suggested that Leonard would have used his quickness to avoid the punch that ended the Baez fight, Hearns became snappish.

"What makes you think Leonard is quicker than me?" he said. "I don't think he is any faster than I am. In fact, I think I am faster than he is. t

"But," he smiled briefly, "I guess we'll find out about that."

Certainly after tonight's brief, easy victory there is still much to be learned about Thomas Hearns.