The defending National Basketball Association champion Seattle SuperSonics, led by Gus Williams' 21 points and John Johnson's 18, defeated Portland, 103-86, today to advance to the Western Conference semifinals.

The 17-point victory allowed the Sonics to beat the Trail Blazers two games to one in the teams' best-of-three miniseries before 23,546 fans.

Seattle now will face Midwest Division champion Milwarkee in a best-of-seven series that opens in Seattle Tuesday night.

The Sonics, beaten at Portland in overtime Friday after a 10-point trimuph at Seattle Wednesday, led 80-65 after three quarters, but the Trail Blazers cut the Seattle lead to nine points at 82-73.

The Sonics met the challenge with a 12-2 spurt in a span of 3 1/2 minutes for a 94-75 lead. Veteran Fred Brown came in the game for Dennis Johnson after he picked up his fifth foul early in the final quarter and contributed three baskets, two of them on 20-foot jumpers, in the spurt.

Williams, Jack Sikma and reserve Wally Walker also contributed baskets as the Sonics moved ahead by 19 points with 4:46 left.

Billy Ray Bates, who joined Portland from Maine of the Continental League with 16 games left in the regular season, topped the Trail Blazers with 26 points, nine in the fourth quarter.

Williams, Seatle's quick guard, who averaged 22 points during the regular season, had nine of his points in the first quarter as the Sonics grabbed a 24-21 lead. The Sonics built that advantage to 57-42 at halftime.

With the score 57-46 in favor of Seattle, the Sonics went on 14-2 tear early in the third quarter for a 71-46 lead. John Johnson had six points in that streak.

Brown finished with 17 points while Sikma had 15 and Dennis Johnson and Wally Walker collected 11 points each for Seattle.

Portland center Tom Owens scored 18 points and picked off 16 rebounds while teammate Kermit Washington tallied 11 points.

The second game of the Seattle-Milwaukee series will be held in Seattle Wednesday night. The third and fourth games will be in Milwaukee Friday and Sunday.

"We just took control," said Sonic Paul Silas, a 36-year-old defensive specialist who has played in more NBA games than anyone except John Havlicek.

"I've never seen guys with such determined looks on their faces when they went out to play. It was really fulfilling to see guys prepared that way."

Seattle held an unusual closed-door practice Saturday in an effort to regroup after after the overtime loss Friday.

"The coaches went back and reviewed the films to see what was needed," Silas said.

What was needed, Coach Lenny Wilkins decided, was more aggressive approach to Portland's outside screens.

"We had five guys ready to give good pressure anytime a screen was involved," Wilkins said. "We're a good pressure team, we play better when we're pressuring."

"I thought Seattle played a very good game, and they deserved to win it," said Jack Ramsay, the Blazer coach. "We have nothing to be ashamed of for our season. When you are competing in the world champion playoffs, you have accomplished something, and that's what I told our players today."