PARIS, JUNE 5 -- To be young was not to be carefree at the French Open today, not in this tournament of world-weary children. Andre Agassi soundly defeated 18-year-old defending champion Michael Chang, while women's second-seeded Monica Seles, tiring at just 16, nearly was upset. The freshest young thing in tennis was 14-year-old Jennifer Capriati, who made history and the semifinals of her first Grand Slam event.

Seles of Yugoslavia is on a five-tournament winning streak, but she was only two points from defeat in the quarterfinals before summoning a last burst of energy against sixth-seeded Manuela Maleeva of Bulgaria, 3-6, 6-1, 7-5. It suggested her stamina is failing, and her next opponent is the unconscionably precocious Capriati, upsetter of Mary Joe Fernandez, 6-2, 6-4, to become the youngest female ever to make the semifinals of a major championship.

A year ago Capriati won the junior title here and was asking for autographs; now her signature is prized. Three months ago she was unranked; now she is No. 15 in the world. A week ago she was unseeded; now she is challenging for the trophy with an untroubled grin. Only Chris Evert in 1971 and Seles last year were able to progress this far in their Grand Slam debuts, also at the French Open.

"I like every minute of my pro career," Capriati said.

Last year Chang became the youngest known men's titlist of any Grand Slam, which consists of the U.S., Australian and French opens and Wimbledon. He was also the first American to capture the French since Tony Trabert in 1955. Now the 20-year-old Agassi of Las Vegas appears poised for a similar feat. The surly, flamboyant star threw his mad modernistic painting of a shirt high into the air after he defeated Chang, 6-2, 6-1, 4-6, 6-2. He is the highest remaining seed at No. 3.

"There's only two guys standing in my way now," Agassi said. "As the tournament goes on, the better I feel. I'm going to fight and sweat and play. It's one of the biggest tournaments of my career."

In the other men's quarterfinal, unseeded Jonas Svensson of Sweden upset crowd favorite Henri Leconte of France, 3-6, 7-5, 6-3, 6-4. In Wednesday's quarterfinals, Thierry Champion of France will play No. 4 Andres Gomez of Ecuador, and No. 7 Thomas Muster of Austria will play Goran Ivanisevic of Yugoslavia.

Seles is on a 30-match winning streak, the longest active among men or women, and over the past month she has shocked both No. 1 Steffi Graf and No. 2 Martina Navratilova in straight-set finals. An unorthodox, left-handed whiplash of a girl who hits the ball with two fists from both sides, she has soared almost as quickly as Capriati, from an unseeded semifinalist in 1989 to the No. 3 ranking. But the string of victories has taken its toll; she is exhausted.

Moreover, Seles' recent defeat of Graf in a tournament in Berlin may have merely angered the West German, who is an unqualified No. 1 and had won 66 straight matches, second-longest in history. While all eyes have been on Seles and Capriati's march toward each other, Graf's advance has been virtually unnoticed. Graf had only moderate trouble with ninth-seeded Conchita Martinez of Spain today, 6-1, 6-3, and her place in the final appears a certainty. Her semifinal opponent will be No. 11 Jana Novotna of Czechoslovakia, who defeated No. 8 Katerina Maleeva, the sister of Manuela, 4-6, 6-2, 6-4, in a duel of clay experts.

Of the attention lavished on Seles and Capriati, Graf said, shrugging, "It is much easier for me that way."

Manuela Maleeva, 23, the sixth seed, meddled with the much-anticipated meeting between Seles and Capriati for 2 hours 4 minutes. She might have ended Seles' tournament if not for a freak injury when she smacked herself in the head with her racket while chasing one of Seles' low winners. The accident occurred as Seles broke serve in the second set for a 2-0 lead with a searing backhand across the court that struck the line.

Maleeva developed a large welt over her left eyebrow and was treated at courtside for more than 10 minutes, a trainer working on her like a prizefighter who had been cut. It may have cost her a second-set blowout. She is now a three-time loser in the quarterfinals here.

"It's swollen, it hurts to blink, I have a headache," Maleeva said. "It was hard. But after, in the third set, I forgot because of all the things going on."

What was going on was a 4-1 lead for Maleeva as Seles' strokes were shallow and lacked their usual precise placement, the Yugoslavian worn down by three consecutive close matches here, not to mention her heavy schedule leading up to this event. Seles climbed back in it with a service break in the seventh game for 3-4, but a game later she stared at elimination, as she trailed 0-30 at 4-5. Maleeva, however, never got a match point.

Seles won the next eight points, beginning the run with two stinging forehands. She ended the surge by breaking Maleeva with a flicked forehand pass off of a drop shot, and held for the match.

"There were no easy games," Seles said. "The feeling of losing never came into my mind, but certainly there were moments and points when I could have lost. . . . There's no question I'm tired."

Capriati is not in the least bit tired; she is spring-legged and brimming with confidence. The seventh-seeded Fernandez is an 18-year-old fellow Florida product who grew up in Miami and lives 40 miles from Capriati's home town of Saddlebrook, but they had not played before. It was a question of two baseliners with heavy pace, and Fernandez proved the more error-prone and yielding.

"Her strategy is just to hit the ball as hard as she can," Fernandez said.

Capriati led by a service break for 4-2 in the final set, gave it back briefly, then broke a second time for the match. Fernandez dug herself into a hole with a double fault on the first point of the final game. She netted a forehand after a slugfest rally to give Capriati match point, and the eighth-grader ended the match by hurling both shoulders into a backhand return winner down the line.

"I think I can win," Capriati said. "Everybody thinks I can win."

Chang was no ingenue. He was sore-hipped and over-burdened. Agassi broke the smaller, less powerful Chang's serve eight times with piercing blows from the baseline. Only Chang's will prevented a straight-set sweep, and for a few minutes in the third set he recalled the form he showed last year when he recovered from a two-set deficit to upset Ivan Lendl in the round of 16.

He used every shot in his repertoire -- slices, drop shots, volleys and looped topspin -- as he broke Agassi's serve twice in succession to claim the set. "I for one wasn't going to fold over after 6-1, 6-2," he said. "I'm the defending champion and I wasn't going to give up."

But Chang has been hampered by the hip he fractured in December, and came to Paris with a season match record of 2-9 and seeded 14th. He is a counterpuncher without a finishing shot; Agassi in the past year has become a thick-chested, well-muscled force. Agassi bludgeoned out a 3-0 lead in the final set, and any hope Chang had of a comeback disappeared when he could not break serve even though Agassi faced a 0-40 deficit in the fifth game.

After his poor recent performance, Chang termed his four victories here encouraging. He also professed relief in defeat, finally shedding the heavy responsibilities of his success.

"Now that I'm out I feel a little weight taken off," he said. "Throughout the whole year, to win a Grand Slam event at such a young age, it felt like a backpack full of bricks." TODAY'S FEATURED MATCHES Court Central

Thomas Muster (7), Austria, vs. Goran Ivanisevic, Yugoslavia; Thierry Champion, France, vs. Andres Gomez (4), Ecuador; Nathalie Tauziat, France, and Judith Wiesner (14), Austria, vs. Claudia Kohde-Kilsch, West Germany, and Brenda Schultz (16), Netherlands.

Tine Scheuer-Larsen-Michael Mortensen, Denmark, vs. Nicole Provis, Australia,-Danie Visser (6), South Africa.Court 1

Louise Field-Simon Youl, Australia, vs. Catherine Suire-Olivier Delaitre, France; Mercedes Paz, Argentina-Arantxa Sanchez Vicario (4), Spain, vs. Nicole Provis, Australia,-Elna Reinach (6), South Africa; Jana Novotna-Helena Sukova (1), Czechoslovakia, or Lise Gregory, South Africa-Gretchen Magers (11), San Antonio, vs. Regina Rajchrtova, Czechoslovakia-Andrea Temesvari, Hungary; Arantxa Sanchez Vicario, Spain-Jorge Lozano (6), Mexico, vs. Rachel Mcquillan-Todd Woodbridge, Australia.