Seattle used its stingy defense-the best in the National Basketball Association - to capture the opening game of its Western Conference championship play-off series with Phoenix tonight.

Game 2 will be played here Friday night.

The Sonics won easily, 108-93, because of strong defensive performances by forward John Johnson and guard Dennis Johnson.

John Johnson shut down Walter Davis, the 6-foot-6 Phoenix forward who finished 10th in league scoring this season with 23.6 points a game.

Davis managed only 13 points and committed seven turnovers. Seven of his points came in the fourth quarter after the game was decided.

Dennis Johnson, named to the all league defensive team, hobbled Paul Westphal, the Phoenix scoring leader this season with a 24-point average. That average was sixth best in the league.

Westphal managed just 13 points and hit only five of 13 shots against Johnson, who also sparkled at the other end. Johnson scored 19 points, grabbed five rebounds and passed for four field goals.

Seattle won the game in the first seven minutes of the third quarter. With center Jack Sikma scoring six points-he had 10 in the quarter-the Sonics built an eight-point half-time lead into a 71-56 margin.

Then in the fourth quarter, Seattle turned the game into a rout behind guard Gus Williams, Seattle's leading scorer during the season. He made 11 points in the final quarter and the Sonics pulled away to a 22-point cushion, 97-75.

Williams finished the game with 27 points, hitting 13 of 26 shots. He also had five assists and three rebounds.

Alvan Adams led Phoenix in scoring and rebounding. The 6-foot-9 center scored 18 points and pulled down a game-high 12 rebounds.

"We knew we had to come out hard, make sure our transition was good and allow no easy lay-ups," said Seattle Coach Lenny Wilkens. "We tried to play good defense on everybody."

Phoenix Coach John MacLead said, "We got off to a bad start. We missed our first six shots, then we didn't play bad for the rest of the first half. We struggled offensively in the second half and at the same time they started to put it all together."

Seattle controlled the game most of the first half because it dominated the rebounding, 28-21, and because of the scoring of Williams and Lonnie Shelton.

Shelton, a 6-foot-8 forward, scored 11 of his 15 first-half points in the opening quarter to help Seattle take an early 10-point lead, 28-18.

Gradually, Phoenix cut into the margin and pared it to 40-36 in the seventh minute of the second quarter.

But Seattle, the Pacific Division champion, quickly shot in front again by a comfortable margin.

Williams, a fleet 6-foot-2 guard, delivered a corkscrew layup, giving him 10 points in the quarter, and Shelton followed with a fast-break stuff that pushed Seattle to a 49-40 lead.