Slightly rusty after more than a week of inactivity, the Boston Celtics surged in the third period when Houston center Akeem Olajuwon picked up his fifth foul and opened the 1986 NBA championship series with a 112-100 victory over the Houston Rockets today at Boston Garden.

Olajuwon led all scorers with 33 points in a game the Rockets let escape from their grasp. Houston center/forward Ralph Sampson picked up three personal fouls in the opening 4:45 of play and sat out the remainder of the first half. He missed his first seven shots of the game, finished one for 13 from the field and scored only two points.

Olajuwon, who scored 25 points in the first half, picked up his fourth and fifth fouls in a 32-second span late in the third quarter. Houston was trailing, 77-72, when Olajuwon was benched with 4:49 left in the period. Boston finished the period with a 14-4 run to take a 15-point lead into the fourth quarter.

The fifth foul was called on Olajuwon for pushing Kevin McHale while both players jockeyed for position near the Boston basket. "As long as it's going both ways, I don't mind," Olajuwon said of the officiating. "But it was very unfair today."

The Rockets had a 65-64 lead on guard Robert Reid's layup with 9:43 left in the third quarter.

From there, the bottom fell out. The Rockets managed only five baskets for the remainder of the eriod and turned the ball over five times. In addition, Olajuwon picked up his fourth and fifth fouls, making it easy for the Celtics to sprint out to a 27-11 end-of-the-quarter run.

In the final 12 minutes of play, Houston could draw no closer than 12 points. The Celtics' biggest advantage came with 6:43 remaining when center Bill Walton dunked an offensive rebound for a 103-82 lead.

Center Robert Parish led the Celtics with 23 points and forwards McHale and Larry Bird had 21 each. Bird also had 13 assists and eight rebounds.

Game 2 will be played here Thursday before the series moves to Houston.

"We were our own worst enemies," said the Rockets' Reid. "It was like we stopped thinking. We started making careless passes and missing the shots we were hitting in the first half. They played great, no question."

With so many talented big men on both squads, the days prior to the start of the series were filled with questions regarding the teams' matchups and strategies. Game 1 provided a glimpse into each team's approach.

The Celtics started the game by isolating Bird, McHale and Parish on one side of the floor in an effort to draw Sampson and Olajuwon into foul trouble. At the other end of the floor, Houston maintained its basic offense, which allowed Bird to sag into the middle, helping the other front-court men on the Rockets' twin towers.

Of course, both teams were forced to make adjustments. Houston fell behind early but came back when forward Rodney McCray -- ostensibly Bird's man -- began handling the ball for the Rockets, moving the Boston forward outside and providing Olajuwon more room to operate down low.

"Just think if he hadn't got into foul trouble," said Reid. "There's no telling how many he would have had."

In tandem, though, Bird, McHale and Parish were creating just as many problems for Houston, scoring in one-on-one situations. The Rockets tried to adjust in the third quarter by beginning to double-team down low, but that just gave guards Dennis Johnson and Danny Ainge more opportunities.

Johnson had five points at the end of the opening half, Ainge just two. They finished with 19 and 18, respectively, and hit 12 of 18 shots in the final two quarters.

"We probably didn't get to . . . rotating on defense as well as we'll have to," said Rockets Coach Bill Fitch. "But if their guards hadn't started shooting like they did in the second half, the game could've been different."

Despite their remarkable record (Boston is 12-1 in the playoffs and has won 39 straight at home), the Celtics have had some close games in the playoffs. Said Walton, who was five for five from the field and added eight rebounds, "We've weathered a lot of storms."

Usually the clouds begin to break at the start of the second half. In the final game of its Eastern Conference semifinal series against the Atlanta Hawks, for example, Boston went on a 36-6 third-quarter blitz to blow the game wide open.

"We talk defense at halftime and transition on defense," said Walton. "In the third quarter, there was no sitting back. Anyone on this team can give us a spark and that's why we're so tough to play against."

Today it was the guards. Johnson scored 12 points in the period, Ainge 10. In addition, Johnson forced a pair of turnovers that led directly to Boston scores.

"This was the first time in the playoffs that we lost our poise," said Fitch. "We started looking for 10-point plays and doing foolish things."

Unfortunately for the Rockets, there was little opportunity for Sampson to get going, given his foul trouble. The hero in the last game of the Western Conference final against the Los Angeles Lakers, the 7-foot-4 pivotman was almost reduced to a rumor today.

"I don't keep track of statistics, so I don't know how well I played today," he said. "I probably wasn't very good, but I know I was frustrated because I couldn't play. The referees took me out of this game, but I can't think about that. I'm already thinking about making a positive contribution on Thursday."

Fitch is certainly counting on that.

" . . . If this team is going to win its first world championship, Ralph and Akeem are going to play a big part of it," he said. "This is a seven-game series. Maybe Ralph has gotten all the bad out of his system.