New York Islander defenseman Denis Potvin surprised no one by saying, "The Rangers surprised all of us-me included."

They are surprising everyone but themselves, it seems. They beat the Islanders, 4-1, in the opener of their National Hockey League playoff semifinal, and took the Islanders through 8:02 of overtime Saturday before Potvin netted a 35-foot shot to give his team a 4-3 triumph.

"The Rangers had an unbelievable level of momentum and overall confidence. It showed for two full periods," said Islander centre Bryan Trotter. "We progressed, slowly, and took the game away."

The Islanders took it away, but not without a struggle. They rallied from a 2-1 deficit to a 3-2 lead on third-period goals 4:22 apart by Bob Lorimer and Bob Nystrom, but the Rangers-showing more poise more often than anyone could have expected-pulled even on Phil Esposito's goal with 4:18 left in regulation.

"We played well, they played well. It was just a matter of who got the break," said Esposito after the Rangers' five-game playoff winning streak had come to an end.

The break came when Potvin attempted to pass to center Mike Kaszycki, but had the puck batted back to him by Ranger defenseman Carol Vadnais. Potkin took a look at the net and shot as Vadnais dropped to block it; the puck glanced off the defenseman and hoped between the legs of goalie John Davidson to tie this best-of-seven series at one victory per team.

Tireless Ranger forechecking bottled up the Islanders' offense through the first two periods. And the Islanders' power play, afforded seven chances by referee Bruce Hood, again was unable to produce-though it led the league in regular powerplay goals.

"Davidson has been terrific," admitted Islander centre Wayne Merrick, "but that's not the reason for our problems. We're frustrated; we're not getting the (scoring) chances we usually get, and there's one big thing we're not getting:rebounds.

"Davidson always stops the first shot, and they always clear the rebounds. We must ge the rebounds. That's the way you score."

"I don't like to look back for cosolation in a loss," said Ranger captain Dave Maloney. "We're happy, now, with the split. We would have been a lot happier with two wins.

"But I can't say this enough times: We're here to play. We're not goint to go home with our tails between our legs.

In the other NHL semifinal series, the Boston Bruins are going home-and are they happy.

The Norris Division champions, who lost their 13th straight game on Montreal ice Saturday night to fall behind the Canadiens, 2-0, in their semifinal series, will have to capitalize on their home-ice advantage when the series resumes Tuesday night at Boston Garden. The last time the Bruins won in the Forum was Oct. 30, 1976 - their only victory in the last 20 appearances there - and since then they have been eliminated by Montreal twice in the finals. In fact, in 17 playoff series, the Bruins have beaten the Canadiens only twice. But both times, it was in the semifinals so there is hope.

The Bruins, health with the exception of veteran captain Wayne Cashman, haven't been a pushover yet, losing, 4-2, in the opener and 5-2 Saturday, and chances are they will be even tougher at home.

On Saturday, Jacques Lemaire, Bob Gainey and Mario Tremblay scored in a 1:24 span late in the second period to wipe out a 20 deficit and power Montreal to victory. Lemaire, who leads the league's playoff scorers with six goals, scored on a power play at 17:41.

"After they took the lead, we were a little disheartened going into the locker room," said Boton goalie Gerry Cheevers. "They have better poise than us. They never lose their poise, even when they're down 2-0."

"We got a goal and that seemed to spark us," said Montreal defenseman Larry Robinson. "But the Bruins came back strong, with (Peter) McNab having a good chance in our end. We came back the other way and scored."