New York Islander fans, who have developed a sense of humor since their heroes won the Stanley Cup, gave Madison Avenue's favorites, the New York Rangers, a rendition of "Ol-la-la, Sasson" in the closing minutes tonight.

The Islanders, who have never been able to beat the Rangers in the endorsement department, and couldn't do it on the ice two years ago, rolled to a convincing 5-2 victory tonight in the opening game of their best-of-seven Stanley Cup semifinal series.

Although the Islanders dominated play, outshooting the Rangers, 31-20, and smothering their offense almost completely in the last 15 minutes, it took a freak goal by John Tonelli to put them ahead for good.

At 6:47 of the second period, two minutes after ex-Ranger Mike McEwen's power-play goal had lifted the Islander's into a 2-2 tie, Butch Goring fired the puck off the rear boards. It came straight out, past Ranger defenseman Barry Beck. Tonelli, skating into the slot, made contact with his skate and kicked it into the net. It was so obviously accidental, however, that the Rangers did not even offer the obligatory complaint to referee Bob Myers that it might have been directed in.

"That was the key play of the game and it was a lucky break for us," said Tonelli, a six-year pro hockey veteran although he only recently turned 24. "I went in to check Beck and I got his stick so he couldn't move the puck. It came right out, hit my skate and went between (Steve) Baker's legs."

Except for a short-handed breakaway by Ron Duguay, on which he skated from left to right across the slot and sent the puck behind the net, the Rangers had few scoring opportunities before Tonelli wrapped it up at 5:45 of the third period.

Some aggressive forechecking by Goring kept the puck in the Ranger end and Tonelli, set up by Anders Kallur, fired from the inner edge of the right-wing circle. The puck hit Baker's pad and trickled over the goal line. Goring, who gave it an insurance tap, originally was credited with the goal, but after the game it was awarded to Tonelli.

"I passed around the net to Kallur and he put it on my stick," Tonelli said. "It got behind Baker and whether I got it or Butch got it really didn't matter. We needed another one and that was it."

Rookie Billy Carroll scored the final goal with 4:37 remaining, but it was apparent only a miracle would enable the Rangers to score twice, so closely were they being checked by the tenacious Islanders.

The Rangers had taken early 1-0 and 2-1 leads as both teams played tentative hockey. There were no whistles for the first 3 minutes 46 seconds of the game, and no shots on goal, as each team went through three lines and had the starters out again before the Islanders were called for offside.

"It was really tight in the first period," Tonelli said. "I don't know if we were trying to feel each other out or what. But this is a big win for us. Two years ago they came in and took the first game off us and I'm sure they were hoping to do it again."

There was some hope in Ranger quarters, as Anders Hedberg produced the only goal of the first period on a power play, both he and the puck being rammed past goalie Bill Smith by Islander defenseman Dave Langevin.

Duane Sutter's deflection of Gord Lane's slap shot tied it 16 seconds into the second period, but Ed Hospodar regained the lead for the Rangers on a pass by Peter Wallin. Then McEwen scored off a faceoff and it was time for Tonelli and the Islanders to get their kicks.