Dan Magnus stands in the corner of the ring at the Rockville gymnasium he owns, sweat streaming down his face and along the scar that divides his chest like a zipper on a sweatshirt.

The scar is a reminder of surgery 20 months ago that nearly ended Magnus' career in professional full-contact karate. But he will be there tonight at the Sheraton Washington Hotel for a match with the second-ranked fighter in the world.

Magnus will face Oliver Miller of Starkville, Miss., for the PKA (Professional Karate Association) light middleweight championship in the main event of a six-bout card beginning at 8. Also featured will be Washington's Lloyd Taylor (20-3), who will take on Manning Galloway (35-8) in a welterweight boxing match.

Few professional athletes have undergone open heart surgery and returned to competition.

Magnus (23-8, six knockouts) gained his current ranking of No. 5 in the world only 14 months after surgeons at the National Institutes of Health opened his chest to repair a tear next to his aortic valve. In his first fight following the surgery, on Sept. 29, Magnus decisioned Tom Dalton, then East Coast light middleweight champion.

"This fight is going to be a real hard fight," said Magnus, 28, about his match with Miller, who has a 24-4 record. "People at the PKA are expressing doubts that I will win. They don't think I should fight. But people love the underdog."

No other fight could be as hard as the one Magnus waged to work himself back into fighting shape. When Magnus weighed in an hour before a fight in Denver against Dalton in April 1983, one of the ring physicians detected a murmur in his heart.

"They told me I couldn't fight," he said. "I could have fought, signed a waiver, but they said I could die. I didn't fight. I went to seven cardiologists and got seven different opinions but they all said one thing -- I couldn't fight anymore. They tested me for two weeks and they couldn't find out what was wrong."

The physicians at NIH discovered the tear a short time later and Magnus immediately asked for the operation. "Right after the surgery, I proved to them (his doctors) that I was going to make it. I was walking around 10 days later, then they told me I had to go home. But two days later, I was back again because of exhaustion from running around and doing things."

Magnus, who is 5 feet 10 and 163 pounds, said he lost almost 20 pounds after the operation and he said he couldn't bench-press four pounds. When he beat Dalton, he had been lifting 200.

"The people at the PKA thought it was a fluke," said Magnus, a Rockville resident who will receive $2,500 tonight plus a percentage of ticket sales. "With this fight, I want to prove it wasn't a fluke."

But fighting for Magnus will not be totally without risk. "Dangerous? It's potentially dangerous," said Dr. Larry Glassman, a medical staff fellow at NIH and one of Magnus' physicians. "It's a dangerous sport for anybody . . . There's a very small risk of developing recurrent damage to the valve through trauma -- through repeated punches and kicks -- not from one punch. But a single fight is not a significant risk."