Bjorn Borg today systematically and relentlessly dismantled Ivan Lendl's forceful back court game and walloped the 20-year-old Czech, 6-4, 6-2, 6-2, to win the $400,000 Volvo Grand Prix Masters Tournament.

Borg -- who at 24 already has five Wimbledon, five French Open, two Italian, and two Masters titles, plus approximately $3 million in prize money -- demonstrated in capturing the $100,000 top prize for the second year in a row that he is back in top form after an indifferent autumn and poised to defend his preeminent position in tennis against all challengers.

The sturdy, 6-foot-2-inch Lendl, who soared into the upper echelon of the game last year and led Czechoslovakia to its first possession of the Davis Cup, can overpower most players with his explosive serve and scorching topspin ground strokes.

He is one of the few players who can occasionally outduel Borg from the baseline, as he proved last October by winning the final of an indoor tournament in Basle, Switzerland, over the Swede, 6-3, 6-2, 5-7, 0-6, 6-4. That was one of four times Borg was beaten in 1980.

Borg was not at the top of his game on that occasion, though. Basle is not the final of the Masters in Madison Square Garden. Today the real Borg showed up, ready to play, and nobody was going to beat him from the back court. Perhaps if they had reincarnated Bill Tilden, or made Rene Lacoste 25 again instead of 75, it would have been a match.

Borg never lost his serve. He saved three break points in the fourth game of the first set, one more in the sixth game, and never faced another. Even though he only got 49 percent of his first serves in court, he lost only 15 points in his last 10 service games.

This has to be a discouraging afternoon for Lendl, as well as for the 18,297 spectators who expected to see a closer contest. Instead, they saw a Borg recital.

Lendl pounded his best shots at Borg time and again in the first set, and saw them come back . . . and back . . . and back.

They had some stirring rallies, trading fierce, deep ground strokes that hissed with topspin. Many points lasted 20, 30, even 40 strokes. One went on for 69 seconds, with practically every shot a screamer. For those first 49 minutes, there wasn't terribly much to choose between them. Little things, mostly. But Borg was slightly steadier and more flexible.

He made fewer errors than Lendl and was more opportunistic, getting to the net to put away volleys on key points.

Certainly, as Borg has said so many times, he played the important points very well. A few examples:

At 2-2 in the first set, after saving the three break points and holding serve after four deuces in the previous game, Borg got to 15-40 on Lendl's serve by picking out the first shortish ball of a grand rally, drilling a backhand approach deep, and swooping in for a gorgeous forehand drop volley. He got the first break of the match moments later, forcing a timid looper from Lendl and booming a forehand winner off it.

At 1-1 in the second set, Lendl faced another break point. He hit a serve that would have been a winner against most players, but Borg returned it deep. Lendl whacked a forehand approach shot to Borg's backhand corner, guessed that the Swede would try to pass him cross-court, and was there to make a solid backhand volley. But Borg anticipated correctly, too, and at full sprint cracked a backhand down the line that whistled past Lendl, breaking his heart as well as his serve.

Two games later, after another backhand down-the-line pass by Borg made it 30-40 on Lendl's serve again, they had another blazing rally, firing topspin bullets at each other. Lendl went to the net behind another strong forehand, this time deep to Borg's forehand corner. Borg may be the only player swift enough to make a play on that ball, and on the dead run again, from a defensive position, he somehow angled the ball back cross-court, inches beyond Lendl's desperate, long-armed lunge.

With experiences like those, it was little wonder that Lendl unraveled. He tried to serve harder, but his accuracy suffered and Borg -- who returned serve magnificently throughout the match -- punished his second serves.

Lendl tried to go to the net more, but Borg was better at that game, too. Lendl tried to blast his ground strokes even harder and closer to the lines than usual, and piled up errors. He lost his rhythm, and even his usually dependable forehand went awry, especially when he tried to whip it down the line from the left court.

After the first set, the deluge. From 2-1 in the second, Borg was home free. From then on, Lendl seemed as ground down as the sawdust he uses in such profusion, reaching into his pocket for a handful with which to dry his palm and racket grip after practically every point. He uses so much of the stuff that ballboys have to sweep up after him at change games, and Borg swept him away just as easily.

"He was just a better player today," said Lendl. He said he was disappointed that he made so many errors, especially on the forehand, but that he wasn't surprised by the quality of Borg's play, even at the net.

Although he felt more intense big-match pressure in the Davis Cup final against Italy last month in Prague, the Masters was the most important tournament final Lendl, the Canadian and Spanish Open champion, has been in. He enjoyed the experience, if not the outcome.

"The atmosphere was just great," he said. "The crowd was supporting both players. I really appreciated that. It is just a big, great tournament. I've only played in New York twice -- the U.S. Open and now Madison Square Garden -- but I like to play here."

The Open is the one big prize that has eluded Borg. He has lost in the final three times, and says that winning it is his major goal for 1981 -- even more important than a sixth French Open or Sixth successive Wimbledon title, though he wouldn't mind those, either.

People said Borg had a New York jinx until he won the Masters last year, and it was evident today that defending the crown meant a great deal to him. At the end, he arched his back and threw up his arms in a triumphant gesture more worthy of one of his torturous Wimbledon triumphs than a 1 hour 57 minute romp over Lendl in the Masters.

"Everybody gets excited when they win, especially a tournament. This was the second time I won the Masters, and the second time I won in New York," he said. "I think maybe this tournament is tougher to win than Paris or Wimbledon or U.S. Open, because you have to play every day, and sometimes you are unlucky and play very late at night and have to come back the next day. You play a tough guy every day, with no days of rest, so maybe it is harder to win."

The Masters -- the playoff for the top eight point winners of the previous year's Grand Prix tour -- is both the finale of the season just completed, and the overture for the New Year. Borg clearly reigned supreme in the tennis world in 1980, and showed in the last week that there is no reason to believe 1981 will be any different.