Martina Navratilova, unbeaten since December, was upset today in the fourth round of the French Open tennis championship by Kathy Horvath, an unseeded 17-year-old who last year considered giving up the game because of back problems.

Horvath overcame a stiff wind and high tension on center court at Roland Garros Stadium to stop the top-seeded woman and her 39-match winning streak, 6-4, 0-6, 6-3.

"I beat Martina, I beat Martina!" she shouted in a telephone call to her parents in Largo, Fla., after the victory.

In another development, John McEnroe was fined $3,000 for misbehaving during a first-round match with Ben Testerman on Wednesday. McEnroe was in a quiet, subdued mood later today when he beat Drew Gitlin, 6-3, 3-6, 6-3, 6-1, in a third-round match and advanced to the final 16.

Also advancing were defending champion Mats Wilander, No. 4 Guillermo Vilas and Americans Tracy Austin, Eliot Teltscher and Jimmy Arias, the Italian Open winner.

Navratilova, last beaten by Chris Evert Lloyd in the final of the Australian Open, blamed her defeat on wrong tactics in the windy conditions.

"I should have driven my backhand," she said. "I played too many sliced backhands, and they didn't carry in the wind.

"I'm certainly not happy about it, but I knew I had to lose sooner or later," said Navratilova, who was beaten only three times last year and lost only four sets in her 39-match streak.

"Losing today certainly has not set a tone for the rest of the year, though. It isn't a disaster for me. The pressure is off now," she said.

Horvath, methodical on court, repeatedly ran to the net and met Navratilova's shots with volleys. Her double-grip backhand volley never failed in the match.

Horvath won four games in a row after trailing, 2-4, in the first set. She was hopelessly outplayed in the second, but in the third she matched everything Navratilova could offer.

"I recovered my confidence in the third and felt I should go for it," said Horvath.

She broke service for 5-3 and served for the match in an electric atmosphere, the French fans cheering every point she won.

She wasted her first match point by netting a weak forehand and on her next match point she drove deep to Navratilova's forehand. The defending champion was caught by surprise and returned the ball out of court.

"She played well, but it was as well as I allowed her," Navratilova said. "I knew she has been playing better lately. People told me her forehand was better than her backhand, but I found out today her backhand is nothing to sneeze at."

The triumph was Horvath's biggest career victory since she first entered the world rankings at 14 in 1980 at slot No. 1,979. Horvath took a four-month break from tennis last year after she developed a lower-back disk problem related to growth and stress while playing at Wimbledon.

She didn't touch a racket for four months and seriously thought about giving up tennis. But after consulting doctors and exercising daily to stretch her muscles, Horvath began playing again last October.

"Kathy decided she'd give it another try," a women's tennis circuit spokeswoman said. "Her long break has made her hungry to win."

Horvath has won only one of 12 tournaments she's played in this year with a won-lost match record of 22-11.

She made it to the final of the German Open on May 16, defeating higher-ranked players before falling to Evert in the title match.

"I think I'm playing better these days because I have a lot of confidence," Horvath said. "I've always had the shots but never had enough confidence."

"I can see the headlines now," Navratilova, 26, joked after the 1-hour, 45-minute match. "It's much more interesting when I get beat by someone like Kathy rather than someone like Chris."

In other matches today, Austin advanced by beating Kathy Jordan, 6-3, 6-1; Jo Durie beat No. 12 Kathy Rinaldi, 6-3, 5-7, 6-1; and Mima Jausovec of Yugoslavia defeated Catherine Tanvier of France, 6-3, 6-3.

Meanwhile, McEnroe, the No. 2 seed, hit brilliant shots in the last two sets of his match against Gitlin. He behaved faultlessly and gave only a puzzled look when he got a questionable line call.

McEnroe's fines were announced by Marshall Happer, administrator of the Men's International Professional Tennis Council after an inquiry that involved videotapes and testimony from witnesses.

He was fined $1,500 for physical abuse--kicking a press photographer's camera--and $1,500 for verbal abuse of linesmen.

The fines brought the total penalties against McEnroe in the last year to $5,750. If he collects more fines in this event and exceeds the limit of $7,500, McEnroe would be automatically suspended from Grand Prix tournaments for six weeks--a ban that would mean missing Wimbledon.

McEnroe is seeded to play No. 1 Jimmy Connors in the final. The men's title has not been won by an American since Tony Trabert claimed it 28 years ago.

Wilander, the No. 5 seed, was his usual cool self and, despite kidnaping threats, he beat France's Dominique Bedel, 6-1, 4-6, 6-4, 6-1.

Threats were made to a Swedish newspaper that a Swedish player in Paris would be kidnaped as hostage for an Armenian being held in Sweden on narcotics charges. Wilander, 18, who has been granted police protection, said: "I am living my life just as before. I am a little worried about it, but it does not affect my tennis."

In other matches, Vilas beat Ilie Nastase, 6-1, 6-2, 6-1; Teltscher, seeded 10th, beat Patrice Kuchna of France, 6-3, 7-6, 6-3; and Arias, seeded No. 11, defeated Marcos Hocevar of Brazil, 6-4, 5-7, 7-6, 6-2.

Jose Higueras, No. 8, beat Jaime Fillol, 6-4, 6-2, 6-4; Henrik Sundstrom, No. 14, beat Joachim Nystrom. 7-5, 6-3, 2-6, 6-1; and Andres Gomez, seeded 16th, beat Shlomo Glickstein, 4-6, 7-5, 6-3, 3-6, 6-4.