The defending National Basketball Association champion Seattle SuperSonics proved tonight they are not ready to give up their crown, pounding out an 86-85 victory over the Milwaukee Bucks to force a seventh game in their Western Conference semifinal.

That game will be played Sunday in Seattle Coliseum. The winner will advance to the conference final and face the Los Angeles Lakers.

Today's victory, before a capacity crowd of 10,938 at Milwaukee Arena, was classic Sonic basketball. They didn't shoot that well, but when they needed a basket they got it. They also rebounded relentlessly and the young Bucks didn't have the muscle or guile to keep up.

A 15-footer by Lonnie Shelton with 1:18 to play gave Seattle an 84-82 lead. The Sonics got the ball back with 1:02 left after Richard Washington missed a shot.

Fred Brown then sank a 20-footer from the top of the key with 45 seconds.

Both teams missed on their next shots. Brian Winters' three-point shot with four seconds left got the Bucks to within a point, but they had no timeouts remaining and never got the ball back.

"We just went out and played like we normally play," said relieved Seattle Coach Lenny Wilkens. "If we lose, we lose."

The game started as if the Bucks were going to snatch the crowns right off the Sonics' heads even though Milwaukee's brilliant sixth man, Junior Bridgeman, was on the bench in street clothes with a back injury. The Sonics turned the ball over five times on their first nine possessions and fell behind, 10-3.

But they settled down, and a 9-0 spurt, spearheaded by Dennis Johnson, got them a 22-20 lead after one quarter.

Brown scored eight points in the first six minutes of the second period and Seattle took control of the game, 35-24.

The Sonics led by as many as 14 points in the period as the Bucks' shots wouldn't fall.

Once it became obvious the Bucks shooters were cold, the Sonics collapsed inside, cutting off Bob Lanier, and let the Bucks have the jump shots.

That strategy worked as Milwaukee made only four of 19 shots in the second quarter and was behind 48-35, at halftime.

The Bucks wouldn't quit, though, and Lanier scored 10 points in the third period. Rookie reserve Sidney Moncrief also gave them a big lift.

The 6-foot-4 Moncrief scored 13 of Milwaukee's 19 points in one stretch to help cut the Buck's deficit from eight to two points, 76-74, with 8:35 to play.

Marques Johnson, who led the Bucks with 22 points, then blocked Gus Williams' shot, resulting in a fast break basket by Winters that tied the score.

The Bucks had numerous opportunities to take the lead in the next five minutes, but botched every one.

Seattle missed its next seven shots and turned the ball over once, but the Bucks still didn't get a single point. In that span, the Bucks missed two shots and Washington and Lanier each missed two free throws.

Seattle's Jack Sikma made his only basket in 14 tries for the night with 5:16 left to put Seattle on top, 78-76.

Marques Johnson tied it at 78 with two free throws, Johnson and Sikma then exchanged a pair of free throws and the score remained tied, 80-80.

The house almost came down when Lanier scored on a drive to put the Bucks on top, 82-80, with 3:06 left, but Shelton tied the game again on a lay up and put the Sonics ahead for good with a 15-footer.