By halftime, Ralph Sampson had decided to score. And once he had the University of Michigan was guaranteed to come out on the short end.

"I figured they were pretty good. "They'd only lost to Ohio State by a couple, so I wasn't sure what was going to happen," the University of Virginia's 7-foot-4 center said tonight after scoring 26 points and grabbing 12 rebounds in the Cavs' 79-68 National Invitation Tournament quarterfinal win before the usual sellout crowd of 9,000 at University Hall. Virginia advances to a Monday semifinal game at New York's Madison Square Garden.

"We seem to psych ourselves out against the smaller teams," explained Sampson, who spent most of the evening making Michigan's 6-8 Paul Heuerman hang on for dear life. "I think that's why it took us a while to get going in the first half. We were intense, but it didn't show up for awhile."

Not for at least for 12 minutes, after which the Cavs were trailing, 22-19, and looking much like the team that had stumbled badly through the last few weeks of the season. Then Sampson and Lee Raker led a 14-2 spurt that gave the Cavs a 33-24 cushion. They expanded that to a 39-28 lead before settling for a 43-35 advantage at halftime. As it turned out, Sampson's 12 points and five rebounds were merely a dress rehearsal.

"I had them figured out by halftime," the freshman star said, grinning. "I thought I could get the ball down low and do whatever I wanted to."

Could he ever. But, perhaps in the interests of competitive spirit, or Michigan's mental health, he waited more than eight minutes before turning theory into reality. After opening the Cavs' scoring with a ringing dunk, he was comparatively quiet until his team's lead and had dwindled to 53-50.

"We were doing all right offensively, but we didn't seem to be flowing," Sampson explained. "I knew something had to break, so I figured it might as well be me."

It was Michigan as well. In the space of the next 2:30, Sampson scored eight of his club's nine points on an inside move, a baseline jumper, an offensive rebound and a hook. When he was finished, so were the Wolverines, who, although trailing only 62-54 with nine minutes left, obviously had been declawed.

As an extra added attraction, Sampson broke the school single-season rebounding record with 351, one more than Bob Mortell grabbed in 1960.

"It's been a while since we played this well offensively," admitted Cav Coach Terry Holland, under fire recently for failing to meet others' expectations for his team.