Playing before a crowd of 17,000 at Roland Garros stadium's center court today, unseeded Mikael Pernfors shrugged off his jitters and then shrugged off third-seeded Boris Becker, 2-6, 6-4, 6-2, 6-0, to reach the semifinals of the French Open.

As a result of his upset, Pernfors will face France's Henri Leconte in one semifinal Friday (Ivan Lendl will play Johan Kriek in the other), and Leconte undoubtedly will own the stadium. Today, he owned everything in sight, including unseeded Andrei Chesnokov of the Soviet Union as he blasted winners en route to a 6-3, 6-4, 6-3 victory.

"In the end it wasn't even a match out there," Becker said of his loss. "He was playing like in a trance. He could do anything he wanted with me. He's 170 [5 feet 8] and he aces me 10 times. That's impossible."

But it happened. Pernfors, a 22-year-old Swede who won two NCAA championships while playing at the University of Georgia, had five match points against Becker last July in Indianapolis in Becker's first match after he won Wimbledon. Becker got out of that match. Today, he had no shot.

When Pernfors ripped a backhand down the line to reach match point, he turned away from Becker and gave himself a lecture, jabbing his finger in the air as he did so. "I was telling myself, 'One more point and then you can jump up and down,' " Pernfors said. ". . . I thought about the five [match points] in Indianapolis. But I pushed it right out of my mind."

And, a moment later, he pushed Becker out of the tournament, scorching yet another backhand down the line past Becker to end the match. The ecstatic crowd had given Pernfors a standing ovation after he hit four straight winners to reach 5-0 and now it stood again.

In the other quarterfinal, Leconte was clicking, and when he clicks he is as wonderful to watch as any player in the world. He is capable of flicking winners in any direction on any shot. Today, his left-handed blasts kept Chesnokov offbalance from start to finish.

"I had no chance the way he was playing," Chesnokov said. "When he is playing like that, I don't know who does."

The margin was impressive in light of Chesnokov's play here. The same player who whipped Mats Wilander on Saturday looked almost helpless today. "I knew I had to get to the net on him," Leconte said. "I watched him play Wilander and I knew I couldn't stay at the base line and hit with him. When I got in, I made it very hard for him."

Chesnokov agreed. "He forced his game on me," he said. "He is the kind of player who sometimes makes impossible shots and then misses very easy ones. Today, he wasn't missing."

Neither was Becker at the start of his match with Pernfors. He whipped through the first set in 20 minutes after Pernfors saved five set points at 5-1.

"I started like usual," Becker said, "Perfect. I couldn't miss a ball for 20 minutes. But in the second he started to get back in it. At 2-2, when I had him 0-40 and let him get away that was very big. If I had broken him there I think I would have won easy in three sets."

Pernfors was in no position to argue. "Someone told me I won five straight points to win the game," he said. "But honestly I can't remember it."

Pernfors broke Becker to win the second set with a cross-court forehand that had Becker rolling in the red clay. But unlike last year at Wimbledon, his dives produced nothing. "Once he won that set he was back in the match and then he just took over," Becker said. "There was nothing I could do."

The last two sets were brief. In the third set, after Becker cut his deficit to 4-2, he did not win another game. Becker has played three Grand Slam tournaments since winning Wimbledon, losing in the fourth round at the U.S. Open, the second at the Australian Open and the quarterfinals here. That is a good record for an 18 year old. But Becker is not judged that way. He is judged as Wimbledon champion.

Pernfors, whose college coach Dan Magill flew in this morning for the match, was ecstatic. "I never thought I could get to the semis when the tournament started," he said. "But when I went out today I thought I could win if I just got him to make some mistakes. When he did, I knew I had a good chance."

Pernfors is different from the other Swedes on the tour. His hair is dark and short. He speaks English with a slight southern accent and as a teen-ager he was unnoticed in Sweden. That's why he went to the United States looking for a college scholarship. He was recommended to Magill by a Georgia player who had lost to him in a tournament.

"When he first came to us," Magill said, "he had no patience on the court. He always wanted to go for a winner. Now, he's patient and waits for the right time."

Today, the time could not have been more right for Pernfors.