Reduced to a mere mortal in the first half by an aggressive band of jump shooters from Virginia Tech, Ralph Sampson played like the 7-foot-4 star he is in the second half and carried Virginia to a 64-51 triumph before 10,056 tonight at the Roanoke Civic Center.

For the second straight game, Virginia (8-0), ranked third in the AP poll and fourth by UPI, had trouble offensively and Sampson was almost forgotten in the first 20 minutes. The Sophomore, averaging 20.3 points and 11.9 rebounds, rarely got the ball inside and managed only 11 points in his team's 53-52 victory over James Madison last week. Tonight, Sampson, who would finish with 19 points and 19 rebounds, scored twice in the first five minutes, then faded into the woodwork.

"Like the Madison game, we were having trouble with our movement and were not getting the shots we liked," said Coach Terry Holland, whose team now has the nation's longest winning streak, 13, in Division I. "We did a much better job of moving and getting the ball in to Ralph in the second half."

And not a moment too soon. Tech, which with its sixth consecutive loss to Virginia fell to 6-4, made up its mind early that it would live or die by the bomb. Of the 33 shots the Hokies took in the first half, 25 were from 16 feet or farther. They made 10 and trailed only 26-24 at intermission.

The Hokies' big man, 6-8 Dale Solomon, tied the contest at 26 in the opening seconds of the second half before Sampson got serious.

He put his team back ahead on a followup shot and, after a free throw by teammate Craig Robinson, who had 16 points, nine rebounds, threw in a dunk. Suddenly, Virginia was on its way, 31-26.

"We just didn't get in the flow of the game in the first half the way we wanted," Sampson said. "We got a bit more aggressive and moved the ball around more. We began to get the shots we wanted."

Tech used its outside shooting to creep to a point, 41-40, with 8:54 to play before Sampson went back to work. Over the next six minutes, Sampson took care of both backboards, scored five points and blocked two shots. Teammate Jeff Lamp, also out to lunch in the first half when he had three points, finally got away from defender Mickey Hardy and scored eight points. Reserve point guard Othell Wilson had four points as the Cavs went on a 17-5 tear to take a 58-45 lead.

"We weren't taking the shots we normally take in the first half," said Wilson, whose four straight points gave Virginia a 52-43 lead. "In the second half we did."

Tech, which made only 21 of 60 shots and was outrebounded, 42-27, didn't have any luck going inside against Sampson and the 6-9 Robinson early in the game. So the Hokies didn't bother trying later, opting instead to shoot over Virginia's zone and hope for the best.

"We knew we would have to shoot it from 15 to 18 feet," said Tech Coach Charles Moir. "It's pretty difficult to drive for baskets against a team that has Ralph under there. He's going to knock them away. We can shoot much better than we did.

"It was pretty obvious Virginia's plan was to get the ball inside to Sampson. He made some key baskets under there, but he also passed the ball well."

Tech's chances of winning were severely damaged when Solomon picked up his fourth foul trying to prevent a Sampson stuff with 10 minutes left. The only Hokie averaging double figures (22.4 points, 11 rebounds), Solomon now couldn't afford to play defense, nor could he play the inside power game he prefers. The junior finished with 14 points and only six rebounds.

The outside shooting of Hokie guards Hardy, 12 points, and Jeff Schneider, 10, was not enough to offset the good second half efforts of Sampson and Lamp.

"We played exceptional defense in the second half," holland said. "We used different types of trapping defenses and they seemed to work better in the second half."

That and a fellow named Sampson were all Virginia needed.