The Philadelphia 76ers, in a bizarre ending to an improbable seven-game series, staved off the Milwaukee Bucks today, 99-98, to advance to the Eastern Conference final of the NBA playoffs.

The first two games of that best-of-seven series with the well-rested Celtics will be played Tuesday and Wednesday in Boston. The crowds are expected to far exceed the 6,704 customers who showed up in the 18,276 capacity Spectrum for today's game.

The 76ers led by 16 points late in the third period today, yet needed four free throws and four rebounds from Caldwell Jones in the final two minutes, and two lucky bounces of the ball in the final 20 seconds, to secure the victory over a team that Julius Erving said "has the best athletes of any team in the league."

Two free throws by Jones with 1:53 remaining put the Sixers on top by 97-93, but with 1:06 left, Milwaukee's Marques Johnson went up over both Jones and Darryl Dawkins to grab an offensive rebound and hooked in a short shot from the lane. It gave Johnson 36 points for the day and cut the lead to 97-95.

Erving, the 76er high scorer with 28 points, missed a difficult shot from the baseline, but the rebound caromed to Maurice Cheeks, giving the 76ers a vital new 24 seconds. With four seconds left on the shot clock, Cheeks' effort was blocked by Bob Lanier, but Jones grabbed the ball and got off a shot with one second left of the allotted 24.

It banged off the front of the rim, but Dawkins snared the ball, meaning the Sixers had a chance to keep possession for the 16 seconds remaining in the game. Dawkins passed the ball out to Jones, who was fouled by Sidney Moncrief with 0:14 to play.

Jones made both free throws for a 99-95 lead.

Milwaukee's Junior Bridgeman sank a three-point shot with nine seconds left to make the score 99-98.

The 76ers tried once to get the ball inbounds and couldn't, so called time. They tried again and failed and called another timeout. On the third try, Bobby Jones threw the ball in to Cheeks. He passed it quickly to Lionel Hollins and the Sixers played keepaway until time ran out.

After the game, Milwaukee Coach Don Nelson read on the official play-by-play sheet that more than 24 seconds had elapsed between the time Erving shot and Caldwell Jones shot on the 76ers' next-to-last possession, which should have brought a 24-second violation call. But review of videotapes showed the play-by-play sheet was wrong and there was no protest.

Erving, who blocked six shots and captured seven rebounds today, called this "perhaps the toughest seven-game series I've been in because Milwaukee blends physical play with finesse so well. And they never give up. They keep coming at you."

Neither team could win two games in a row in this series and it took a couple of key coaching decisions by Philadelphia's Billy Cunningham to give the 76ers the edge they needed today.

Friday night in Milwaukee, the 76ers were outrebounded, outhustled and routed by 17 points. Today, they came out with a half-court trapping defense, and on offense, they put Erving on one side of the floor and his teammates on the other and let him work one on one.

"Why fool around? We want Doc with the basketball, so why not just give it to him?" said Cunningham. "He either gets the shot, or creates something for somebody else."

With Erving enjoying every minute of that offense and reserve Bobby Jones coming scoring eight of his 21 points in the third quarter, the 76ers took control at 74-58 with 4:42 left in the third period. But then the Philadelphia offense stagnated. The Sixers missed their next eight shots, Milwaukee scored 10 straight points and by period's end the Bucks were down by only 79-73.

The visitors started the final quarter with a 7-0 spurt to take an 80-79 lead and the game was up for grabs.

"Before the game I told Billy I wished the best team would win," said Milwaukee Coach Don Nelson. "I'm not sure it did."

Milwaukee's troubles started early as both Mickey Johnson, the hero of the sixth game, picked up three quick fouls and Quinn Buckner, playing with a severe stomach ache, picked up his third a short time after. Both went scoreless for the day.

In fact, only Marques Johnson, Lanier and Moncrief scored in the first quarter for the Bucks. Brian Winters was the only other Buck to score in the second period and Bridgeman the only other to score the rest of the game.

Steve Mix and Lanier exchanged punches in the second quarter, but neither landed a blow and neither was ejected.

Cunningham remarked on the Easter turnout: "I think the attendance merely reflects how Philadelphians feel about the 76ers.

"It's a shame, isn't it?"