Leonardo David, an 18-year-old star on the Italian ski-racing team, was listed in critical but stable condition tonight after a two-hour brain operation following his crash near the finish line of a World Cup downhill race.

David lost his balance on a bump about 50 yards from the finish line.He slid alternately on his side, back and stomach across the finish line, apparently striking his head on the snow in the process.

But he quickly got to his feet unassisted, only to collapse after skiing over to one of his coaches and bending over to take off his skis.

He was rushed to Burlington, Vt., by helicopter and surgery was performed in midafternoon. Doctors relieved pressure from an internal hemorrhage.

David's crash occurred before thousands of spectators on a warm, sunwashed day in an international ski race won by Austria's Peter Wirnsberger. The race took place on the same hill that will be used for the 1980 Winter Olympics.

At a press conference tonight in Lake Placid, a report was read from Dr. Henry Schmidek, who performed the surgery.

"David, when he arrived at the hospital, was very grave," the report said. "He had a big internal hemorrhage. There was extensive pressure in the head.

"We have operated and we took out the coagulations. There are no other signs of problem in the head. His condition is stable."

David was given barbiturates to induce continued coma, according to the Italian men's team coach, Josef Messner.

"We won't know for five to seven days what his condition is," Messner said.

David was on his first World Cup circuit this year. He won the 1978 Europa Cup and was described by U.S. Alpine Director Hank Tauber as "undoubtedly the best male racer to burst upon the international scene since Phil Mahre won the Val d'Isere World Cup giant slalom in 1976 at age 19."

David wore bib No. 19, which meant he crossed the finish just after the eventual winner, Wirnsberger, and No. 2 finisher, Peter Mueller of Switzerland, had completed their runs.

The corral was buzzing as people pressed to congratulate the leaders. David went down in a heap. "He was looking at me," said a staff worker. "Then he just fell."

His collapse marred what to then had been a perfect day of ski racing. Temperatures soared into the 50s and spectators stripped to turtlenecks and T-shirts as the finest ski racers in the world made their descents.

He was unattended for only a matter of minutes after he collapsed. Dr. Howard Smith of New Haven, Conn., working for the Lake Placid Olympic Organizing Committee, was summoned from a post near the finish.

Smith performed artificial resuscitation and opened a track in the throat with a plastic device. He later accompanied David on the helicopter trip.

The only delay came when a sled was unavailable to transport him down the mountain. It arrived in about 10 minutes. David was carried to the helicopter pad.

Messner said the Italian team had no complaint about the handling his skier received.

David fell in his last race, Feb. 16, at Cortina, Italy. Messner said his skier complained of headaches after the fall, "but he passed all the tests after that." He had not complained of any headaches while at Lake Placid for practice runs the last three days, Messner said.

He said that five minutes before the race David was "in good shape."

The winner, Wirnsberger, played the hill to advantage after near missing the start altogether.

"The starter, he counts for the skier," Wirnsberger said after his second World Cup downhill victory of the year. "Five, four, three... Only this time the five, four, three, he forgot by me."

What that meant was that the 21year-old Austrian was still waiting at the start after the countdown period ended. If he had waited three seconds, he would have been disqualified. But he come to his senses and pushed off in a panic with eight-tenths of a second to spare.

He lost nothing on elapsed time because the timing clock began when he left the gate. "But I lost something on readiness," Wirnsberger said.

Not enough to cost him. Mueller was smooth and fluid but he finished three-100ths of a second behind for the two-mile run. Canada's blond bearded David Murray was third, another eight-100ths back.

The day did little to buoy U.S. hopes for next year's Olympics, which will be run on this same course. The best U.S. finisher was downhill specialist Karl Anderson, 14th.