The Philadelphia 76ers overcame a 13-point second-quarter deficit and defeated the New York Knicks, 112-107, yesterday in Madison Square Garden to win their NBA Eastern Conference semifinal playoff in four straight.

The Knicks were badly outclassed in the series and while they played better yesterday than they did in the three previous games, they didn't have the firepower offensively or defensivly to keep up with the 76ers.

Philadelphia will play the winner of the Washington-San Antonio series for the Eastern championship. The 76ers will have the home advantage.

Doug Collins posted 24 points and Julius Erving 23 yesterday, but the 76ers were not their usual runnin'-gunnin' selves.

Knick rookie Ray Williams scored six straight and then Bob McAdoo four in a row to give the New Yorkers a 50-37 lead with 5:39 to play in the second quarter.

Philadelphia called time and instead of becoming more offensive-minded, clamped down on defense and controlled the boards.

Erving scored nine points in the period's final five minutes and the Sixers were down by only 59-57 at intermission.

"This series showed us that defense and defensive rebounding can keep us in games when the offense isn't clicking," Philadephia Coach Billy Cunningham said.

After the Knicks took that 50-37 lead, they missed nine of their next 11 shots under heavy pressure. They also turned the ball over three times. Philadelphia converting each miscue into a basket, one by Joe Bryant, one by Erving and one by George McGinnis.

The lead seesawed most of the second half. Philadelphia pulled away in the final four minutes.

their usual runnin'-gunnin' selves.

"This series showed us that defense and Spencer Haywood tied the score at 97 on a turnaround jumper with 3:55 left. Then New York didn't score again for 2 1/2 minutes, while Philadelphia ran off five points to put the game away.

After Haywood's basket, Erving was called for an offfensive foul, but the Knicks couldn't convert on their next possession. Then Lonnie Shelton fouled Philadelphia's Lloyd Free as Free drove toward the basket.

It had been a frustrating afternoon for Shelton and that was his sixth personal. He showed his anger by flinging the ball downcourt and drew a technical.

Erving made the techincal shot, then Steve Mix scored on a drive and the 76ers were home free.

"We're just a better team than the Knicks,' said Erving. "We have a deeper bench and every change they made we had an adjustment for. The Knicks are a good team of offensive individuals, but unless they are clicking with their zone, they have problems defensively."

McAdoo led New York with 24 points and Earl Monroe helped with 19, but that pair scored only four points apiece in the final quarter.

"It's no embrarrassment to lose four straight when you put out all you can," said Monroe, "and we did. We were just too inconsistent."

Collins, who had five rebounds and four assists, said, "Sure we wanted to win in four straight. We don't want to give anybody a chance. It's tough to sweep a team, any team, but we're on to win every game we play. We let an opportunity slip away last year and we're not going to let that happen again."

There was one unhappy 76er, McGinnis Reserves Mix and Bryant were playing well, McGinnis was not, and thus he was employed only 21 minutes. He did not play at all in the fourth quarter.

McGinnis was fuming, and Cunningham said, "I know what a great player George McGinnis is, but other guys were doing better. It's not my job to worry about how players feel about not playing, it's my job to go with whatever combinations accomplish what I want to accomplish."

McGinnis made three of none shots, scored 10 points and took seven rebounds. Mix contributed 12 points and three rebounds and Bryant eight points, five rebouds, three assists and two steals. Mix and Bryant also played tougher defense than McGinnis.