The Boston Celtics flexed their muscles today and served notice tha this National Basketball Association championship series may be a joke after all.

A relentless defense exposed all of the flaws in Houston's slowdown, conservative offense as the Celtics routed the overmatched Rockets, 94-71, at the Summit ot take a 2-1 lead in the best-of-seven series.

Houston's pathetic point total tied the record low for an NBA final series. It was lucky to score that many. Seldom did a Rocket get an open shot and, when one did, the shot usually clanked off the glass.

The Celtics led, 65-48, after three periods. With the end-of-bench reserves playing most of the final quarter, the Rockets managed 23 points in the last 12 minutes to sneak into the 70s.

The Rockets shot 30 percent in the game and committed 22 turnovers, 10 more than they have averaged in the playoffs. There were times today when they looked like they had no idea what to do, with Celtics standing everywhere they wanted to go and a Celtic arm in every passing lane.

"We just have to erase this whole game," said Houston Coach Del Harris, "because if you judge us on this one, it looks like we need to go back to training camp and learn what this game is all about again. But I know we're better and we'll be back tomorrow."

The fourth game will be Sunday afternoon, also at the Summit (WDVM-TV-9 at 1:05). The fifth game will be Tuesday night in Boston (WDVM-TV-9 at 8:05).

The Celtics stunned the sellout crowd of 16,121 with their wholesale dismantling of the home team.

"We applied great defensive pressure and wouldn't let them get started," said Boston guard Chris Ford, who scored 17 points, eight of them in the first period, as the Celtics got off to a 21-17 lead. tt"We didn't practice Friday, but had a two-hour meeting, instead, and decided that we wouldn't let them have an easy pass anywhere. In the first two games, we were letting them throw the ball wherever they wanted to."

The Rockets know their only hope in this series is to control the tempo by slowing play. They usually try to use up most of the 24-second clock on each possession.

The Celtics came out with full-court pressure today, which, ironically, slowed the Rockets down more than they wanted. That disrupted their offense. s

"They weren't shooting their shots in their usual patterns and out defensive pressure gave them less time to set up, so they couldn't get into a rhythm," Ford said.

The Celtics, who turned the Houston turnovers into 29 points, played their game throughout, running whenever they had a chance. They outscored the Rockets ont he break, 41-7.

Boston had six players in double figures, led by Cedric Maxwell's 19 points. Larry Bird had only eight, making three of 11 shots, the first time in 23 career playoff games he hasn't scored in double figures.

"Some days, you're goint to hit shots and some days you're not," Bird said. "Lucky for me, I can do other things with the basketball besides just shoot it."

Indeed.

Bird collected 13 rebounds, had 10 assists -- equaling the team total of the Rockets -- and had five steals.

Moses Malone was high scorer for Houston with 23 points, but Bill Willoughby was the only other Rocket in double figures with 12.

The second quarter was when the Rockets died today. They made only three of 17 shots and scored 13 points. Malone had nine of the points, weven of them from the foul line.

"We started trying a bit too hard that period," said Calvin Murphy, who missed three shots in the quarter. "Then we started pressing and got into real trouble." Murphy turned 33 today but he wasn't celebrating.

Robert Parish, who fouled out after 14 minutes in the second game of the series, was in early foul trouble again today and played only 11 minutes. He got three fouls in the first 5 1/2 minutes, sat out the rest of the half and picked up his fourth and fifth fouls in the first 4 1/2 of the third quarter.

The Celtics led, 45-36, when Parish was called for his fifth foul going for a rebound, but before Coach Bill Fitch could get him out of the game, Parish scored seven straight points, to stake the Celtics to a 52-36 lead.

"I wasn't going to let the calls frustrate me," Parish said of his scoring burst, accomplished in 72 seconds. "I was just trying not to let it get me down.

"I have to make the adjustments if I want to stay in the games. I have to play better position defense and keep my hands off Moses because when I put my hands on him is when they call a foul."

Parish also put his head on Moses once, resulting in five stitches over Parish's right eye.

The Rockets closed the third period with baskets by Murphy and Malone to cut a 21-point deficit to 17, but Boston ended the teaser by opening the fourth quarter with six straight points to go ahead, 71-48.

"We just didn't have anyone to pick us up," said Houston's Bobby Joe Reid, who is known as Robert Reid everywhere except in Houston. "They beat us nside and they beat us outside. Our frustration started building and we tightened up. I'm glad we get to play the next game so soon."

So are the Celtics.

"We plan to make Sunday's game a continuation of today," said Ford.

"We'll come out with the same intensity and keep the defensive pressure on. We're through fooling around."