The Philadelphia 76ers are in trouble.

After taking a 3-1 lead over the Boston Celtics in their best-of-seven National Basketball Association Eastern Conference championship series, they have blown big leads the last two games, allowing the Celtics to tie the series at 3-3 and force a deciding seventh game Sunday at Boston Garden (WDVM-TV-9, 3:30).

Friday night, the 76ers held a 17-point lead in the second quarter and lost, 100-98. It was the first time in 12 games Boston had won in the Spectrum.

"We have two choices," Philadelphia Coach Billy Cunningham said. "We can hang our heads or we can go into their place and beat them. It's up to the players."

"We have to be able to dominate the boards if we're going to beat them," said forward Bobby Jones. "In Friday's game they closed everything off inside and that stopped our offense. We just have to win Sunday. That's all. There no momentum or psychological advantages or any of that. We're all even and the best team will win."

Darryl Dawkins, the man the 76ers say has to play well for them to win, has played well the last two games. But the Sixers still have lost. Dawkins led his team with 24 points Friday and was unstoppable once he got the ball inside.

"I didn't want to go back to Boston," Dawkins said. "But I told everyone when we went ahead, 3-1, that this thing wasn't over and it still isn't over."

The Celitics, who celebrated as if they had just won the world championship with Wednesday's victory in Boston, were more subdued Friday night.

"It's tough coming back from 3-1, but this team has a lot of character," said Larry Bird, who had 25 points, 16 rebounds, four assists, two steals and two blocked shots in Game 6. We'll never quit no matter what the score is or how late it is in the game."

Friday's game was by far the roughest of the series. Sixty-four fouls were called and twice as many could have been. Neither team gave up anything without a struggle.

On the Boston basket, seven bodies were strewn on the floor as Bird grabbed an offensive rebound and scored before going down himself.

"You think that game was physical, wait until Sunday," said Boston center Robert Parish.

Tiny Archibald gave the Celtics a scare when he twisted his foot on a drive to the basket in the third period, but he returned and had two big baskets late in the game.

"You can't give anybody anything," he said. "The offense is there and the defense is there. We're playing a great team that never quits. We're a great match up for each other. That's why this series is going seven games."

History is on the side of the 76ers. Only four teams have come back from 3-1 deficits to win a seven-game series. The last to do it was the Washington Bullets, in the 1979 Eastern Conference championship series against San Antonio.

Sunday's winner will play the Houston Rockets for the NBA championship in a seven-game series beginning Tuesday in the Eastern city. Thursday's second game also will be in either Boston or Philadelphia, with Games 3 and 4 in Houston next Saturday and Sunday.