Fred Brown's 10 third-period points pushed Seattle on top and Gus Williams added 10 free throws in the final quarter to lead the SuperSonics to a 106-100 victory over the Los Angeles Lakers tonight. The win gave Seattle the National Basketball Association's Western Conference semifinal playoff series, 4-1.

The Soncis will defend their Western Conference title against the winner of the other conference semifinal, Phoenix or Kansas City.

Brown replaced Williams early in the third quarter and scored six straight points to cap an 11-2 Sonic rally for an 82-81 lead at the period's end. Brown's short baseline jumper with 26 seconds left in the quarter put Seattle ahead for good.

Williams, who led all scorers with 30 points, replaced Brown to start the final quarter. He scored eight points in a 12-2 Seattle run that put the Sonics in command, 94-83, with 6:25 to play.

The Lakers couldn't seem to buy a basket during the first six minutes of the final quarter. The Sonics forced Laker center Kareem Abdul-Jabbar outside, and he misfired on several turnaround jumpers from outside his range.

The Lakers ran off a 12-3 run in the final three minutes of the second quarter, including four points each by Abdul-Jabbar and Ron Boone, for a 59-53 halftime lead. They built their margin to 69-59 on a pair of Boone baskets with 8:35 to go in the third quarter.

Then Brown went to work from outside as well as from the baseline to slowly close the gap.