PARIS -- Mary Pierce, a transatlantic teenager with a blonde ponytail, a French mother and an American father, stunned the sports world with a dazzling display of searing ground strokes that devastated world No. 1 Steffi Graf and shattered her monopoly of women's tennis.Pierce's startling upset Thursday of the German star in a 6-2, 6-2 victory in the semifinals of the French Open deprived Graf of a chance to win her fifth straight Grand Slam title. It also gave a ringing response to skeptics bemoaning the plight of women's tennis by declaring a new challenger has arrived to threaten Graf's domination of the sport.Pierce, the No. 12 seed, will now become the first French woman since Fran?oise Durr in 1967 to play in the finals of the world's greatest clay-court tournament. On Saturday, she will confront Arantxa Sanchez Vicario.Ever since Monica Seles was forced into premature retirement after being stabbed by one of Graf's admirers at the Hamburg Open more than a year ago, the absence of any serious rivals has left Graf looking bored as she walloped every opponent, usually in less than an hour, and embellished her trophy room.Pierce, showing a blissful serenity on the court, has dropped only 10 games in the six matches she has played. She has injected new power and stamina into her game following an intensive program of weight lifting and aerobics conditioning.The Canadian-born Pierce learned her tennis growing up in Florida from her strong-willed father, Jim. He has now been banned from attending her matches because of violent outbursts in the past. Since splitting with her father, Pierce has found a restored sense of comfort and humor since adopting Nick Bollettieri and Sven Groeneveld, a Dutchman aged 28, as her coaches.Bollettieri, whose famed academy has molded Andre Agassi, Jim Courier, Pete Sampras and Seles into precocious stars, has honed Pierce's physical condition and infused new aggressiveness into her game in recent months.