The Philadelphia 76ers made up a 20-point deficit in one marvelous nine-minute stretch of the third period tonight and stunned the New York Knicks, 98-91, at the Spectrum to take a 2-0 lead in their best-of-seven NBA Eastern Conference semifinal playoff series.

The 76ers were playing without all-star guard Andrew Toney, who had a bruised thigh, and forward Bobby Jones, who had the flu. After falling behind, 63-43, with 10 minutes left in the third quarter, it looked like a rout.

But the Knicks fell apart and the 76ers, led by guard Maurice Cheeks--at 6 foot 1 the smallest player on either team--came on in a rush.

The bewildered Knicks tried everything imaginable to stop the onslaught that resulted in a 22-1 run by the 76ers. The Knicks were held to nine points in the third quarter and went without a field goal for 9:45 in the quarter.

During the run, New York missed 14 shots and committed nine turnovers. Coach Hubie Brown called timeouts, made substitutions and screamed at the officials, his players and his assistants. Nothing helped.

"It was a beautiful thing to see," he said. "Even if it happened to us."

Truck Robinson, who led the Knicks with 22 points, scored on two moves inside early in the period, but then the Spectrum roof caved in on them. Every place they turned, Cheeks seemed to be there, either stealing the ball, forcing a turnover or breaking up a play. He had three steals and nine points in the quarter and finished with 26 points.

Julius Erving made a rare fast-break basket by Philadelphia to cut New York's lead to a point and Franklin Edwards put the 76ers ahead, 65-64, with two free throws.

New York scored the period's final four points, but the 76ers opened the final quarter with an 11-2 rally. That gave them a 76-70 lead and the Knicks weren't much of a threat the rest of the game.

"It's hard to believe a team can play such different halves," said Philadelphia Coach Billy Cunningham. "In the first half, we weren't aggressive defensively. Without Andrew and Bobby, I was concerned about fatigue and so we picked them up at midcourt instead of full court like we usually do.

"After the half, though, we went full court. Our defense is what makes everything work for us."

The 76ers started Clint Richardson in Toney's place, but he went one for six from the field and Edwards one for eight. They missed Jones just as badly as they did Toney, though, as they had no one who could guard Robinson, Bernard King, or any of the other New York forwards in the first half.

Robinson, King, Sly Williams and Louis Orr accounted for 38 of New York's 59 first-half points as the Knicks led by 17.

In the second half, Cunningham put Moses Malone on Robinson and double-teamed at every opportunity. New York, which is not a good ball-handling team, couldn't handle the pressure.

The Knicks' 7-footers, Bill Cartwright and Marvin Webster, were so intimidated by Malone they took six shots between them and made one. Cartwright went scoreless for the first time in his pro career.

Malone led the 76ers with 30 points and 17 rebounds; Erving had 20 points. Cunningham used eight players and only six scored.

Game 3 will be Saturday at Madison Square Garden. Cunningham expects both Toney and Jones to play.