Monica Seles, the world's top-ranked female tennis player, was stabbed in the back Friday and slightly injured by a spectator while she was playing a match in Hamburg.

Seles, 19, was sitting during a changeover in her quarterfinal match against Magdalena Maleeva of Bulgaria when she was stabbed in the back by a man wielding a 9-inch boning knife.After Seles was taken to a hospital, doctors said she had a slight muscle tear, which will cause her to miss three to four weeks of tournament tennis. Seles, who this week was returning to competition after being sidelined two months by a viral infection, will now likely miss the French Open, the Grand Slam tournament beginning May 24 that she has won the past three years.

After the stabbing at the Rothenbaum Tennis Club, security guards immediately restrained the attacker, then turned him over to the police, who rushed him away from the stadium. A police spokesman later said the man was a German citizen, 38, from Thuringia, a state in eastern Germany. Police would not release his name.

A police spokesman, Dankmar Lundt, told a German television station said the man is German and said "there were no political grounds" for the attack. Lundt said the man "appears confused" and might be mentally disturbed.

"He said he was a Steffi Graf fan," Lundt said, referring to the German player ranked No. 2 behind Seles. "He didn't want to kill Seles, only injure her to hinder her from playing."

It is believed that no tennis player had ever been attacked during a match.

Gerry Smith, chairman of the Women's Tennis Association, which oversees the professional tour, was in Hamburg.

"I think it's going to have a very dramatic effect," Smith said. "It's going to change the psyche of everyone associated with the sport. We've had threats before and tried to take all possible precautions. Somehow it seemed remote until this moment. It's no longer a threat; it's a reality."

Seles is an ethnic Hungarian who was born in Novi Sad, the capital of Vojvodina, an autonomous province in the former Yugoslavia claimed by Serbia. She has been the target of death threats over the last two years related to the strife in her homeland. But she had received no threats in Hamburg.

Seles has repeatedly expressed her view that sport and politics have nothing to do with each other. She has lived in Florida since 1986.

The incident occurred during a changeover in the second set just after Seles, who won the first set by 6-4, had recovered from an 0-3 deficit to lead by 4-3. Both players were sitting in their chairs on the side of the court. There were two security guards stationed behind each player because of the proximity of the players' seats to a pedestrian aisle at this stadium, which seats only 8,000.

A balding man of medium height in a plaid shirt and jeans walked down the pedestrian aisle. Eyewitnesses said he appeared drunk or deranged, certainly strange. When he got to the waist-high barrier behind Seles, he raised the knife with both hands. A woman screamed at that moment, and Seles half turned.

The blow came at an angle between her shoulder blades, and Seles leaped to her feet and walked toward the center of the court, holding her back with one hand and screaming. The chair umpire, Stephane Voss, and a spectator assisted her on the court, and her brother Zoltan joined them.

Meanwhile, the assailant dropped the knife on the court and was restrained by the security guards, who reportedly broke his arm in the process.

Seles remained conscious and was sitting up as she was wheeled out of the stadium on a stretcher. An ambulance took her to Hamburg's Eppendorf University Hospital.

After examining Seles, tournament doctors Andreas Witthofft and Peter Wind said she had "suffered a 11/2-to-2-centimeter wound in the back area without damage to any vital organs." They said that the knife narrowly missed her spinal area.

"She was very lucky," Wind said. He added Seles was also suffering from a psychological shock and would remain in the hospital overnight.

Seles has been the most dominant player in women's tennis for the last two years. In March 1991, Seles at 17 surpassed Graf for the top ranking and became the youngest player, male or female, to be No. 1. She has won eight Grand Slam tournaments. She reached the final of the every event she entered in 1991 and compiled a 70-5 match record in 1992.

Hamburg tournament officials defended the security measures at the event and said they would continue play.

"Our security measures are no different from other tournaments," said Jens-Peter Hecht, spokesman for the German Tennis Association. "There are no body searches like at football matches."

In the aftermath of the Seles stabbing in Hamburg, the U.S. Tennis Association issued a statement about security at the U.S. Open: "Even though the U.S. Open security system is complete in all respects, the USTA will, as a result of this incident, re-examine all security measures and, if necessary, further tighten its security so that all players and fans can enjoy and feel secure at the 1993 U.S. Open."