The New York Islanders carried the National Hockey League's best record into the opener of their Stanley Cup semifinal with the New York Rangers tonight and played like the 12-victory expansion team of six years ago.

The Rangers outskated, outhit and outplayed the Islanders over the last 50 minutes for a startling 4-1 triumph in a game so important to New Yorkers that some were paying $150 for single tickets outside Nassau Coliseum.

Enough Ranger fans were inside to mount unchallenged chats of "J.D." in honor of goalie John Davidson, who made some big saves among a meager total of 21 as the Rangers recorded a 38-22 shooting edge.

The key to the Ranger victory was their ability to clear the defensive zone without difficulty.

"I'm not sure exactly what we were doing right," said Ranger Coach Fred Shero, "but we were coming out easily. It was a matter of timing. They always seemed to arrive a second too late."

"We didn't pay well," said Islander defenseman Denis Potvin without creating any argument.

"It was like we were out for a practice rather than a game. I think it was a mistake in preparation. We thought we could beat them like Chicago, without working too hard. Now we know we have to work damn hard to get anything from them."

Mike Bossy, the Islanders' 69-goal scorer, did not get a shot tonight and Shero, advised of that statistic, said, "Well, it's not my fault."

With Islander Coach Al Arbour geting the last change, Shero simply let any line that was due for a shift go out against Bossy, Bryan Trottier and John Tonelli, with Clark Gillies replacing Toneilli in the last 30 minutes. But Shero made certain that Dave Maloney and Carol Vadnais, his most experienced defensemen, went against the big line.

"We have the momentum and we're playing disciplined," Maloney said. "We've learned that we can't become intimidated. We know we can beat these teams. There's a lot of talent in this room."

The Islanders had won eight straight since that April 1 loss in Washington and had dropped only two of their last 21 games. So when Trottier sent them in front at 10:41 of the first period, a lot of people thought the game was over. The Islanders apparently did, too.

Don Murdoch tied it on the Rangers' first power play, with Gerry Hart off for holding, and before the period ended Eddie Johnstone had put the Rangers ahead to stay on a rebound of a Vadnais slap shot.

The shots on goal in the first period were surprising, the Rangers testing Chico Resch 14 times and the Islanders managing only eight on Davidson. The second-period shots were downright unbelievable, the Rangers again colecting 14 while the Islanders were limited to three.

Ron Duguay made it 3-1 off a fine drop pass from Pat Hickey and Bob Sheehan converted Pierre Plante's pass out of the corner, after Plante had robbed Bossy of the puck, to close the scoring.

Sheehan, a 30-year-old Massachusetts native, had been stuck in the American League for two years until Shero, who doubles as general manager, called him up the morning after the Rangers had dropped the opener of their quarterfinal series with Philadelphia.

Neither Sheehan nor Johnstone has played in a losing playoff game. Johnstone, who has five goals in seven games, matching his regular-season total, did not dress for the loss in Philadelphia. CAPTION: Picture, John Davidson