As a child, she yearned to be just like Pedro Martinez, a pitcher supposedly too fragile and slight of build to make it big in the major leagues but whose next stop might be Cooperstown.
At age 10, only a fourth-grader, she was on " Jimmy Kimmel Live," striking out the late-night host with a wicked off-speed pitch.
By 14, she was in a Nike commercial, stepping up to a giant megaphone and earnestly declaring that she someday wanted to play for the Boston Red Sox.
Hers is a Cinderella story in which the slipper might actually fit — size 5, with spikes.
Her name is Marti Sementelli, and she doesn't care that some of the varsity high school players she pitches against are a foot taller and 100 pounds heavier than she is.
"I never really doubted myself," the right-handed reliever from Lake Balboa Birmingham High said.
Why would she? Now 17, the 5-foot-2, 118-pound junior has allowed only one run in her last seven innings, helping her team rebound from a 2-24 season to make the City Section playoffs. Birmingham (13-13) plays at Kennedy (13-12) in a first-round game on May 19 at 3 p.m.
A highlight video of Marti's career posted on YouTube spans her career, her size and uniforms changing but not the leg kick and three-quarter arm whip that have been trademarks of her delivery from a very young age.
The high school playoffs mean extra pressure for some, but Sementelli has already excelled on an even larger stage. Two years ago, she was selected top pitcher of the Women's Baseball World Cup after helping the United States defeat Australia in the bronze-medal game.
That prompted her father, Gary Sementelli, to gush that his daughter might just be the best female baseball pitcher on the planet.
Sloan Boettcher, a first baseman who batted .222 for Palos Verdes High this season, is believed to be the only other high school girl currently playing for a varsity team in the Los Angeles area.
Birmingham is part of the vaunted West Valley League, which includes perennial powerhouses such as Chatsworth and Woodland Hills El Camino Real, so Sementelli is doing more than playing in the vaunted West Valley League; she is competing.
She "is facing some of the best competition you could possibly face and holding her own," Birmingham Coach Matt Mowry said.
Short and slight, with her past-the-shoulder hair tucked neatly under her cap, Sementelli is not at all imposing, but she competes as hard as anyone.
"There's parents in the stands [and] guys on the other team who are like, 'Who's that? It's a small little dude,' " Birmingham catcher David Rodarte said. "After a while, they see her and recognize that she's a girl because her name's getting around pretty quick. Everyone's just starting to realize who she is — Marti Sementelli, a good pitcher."
Sementelli's repertoire includes two kinds of palm balls, a changeup, a curveball, a slider and a fastball that tops out in the mid-70s. She gets outs, Mowry said, by changing speeds, throwing strikes and keeping the ball down in the strike zone — same as any good pitcher.
Sementelli has a record of 2-1 with a 5.67 earned-run average and 14 strikeouts in 27 innings. She picked up her first victory March 20 with three innings of relief against Harbor City Narbonne and notched a second triumph 10 days later by pitching six solid innings in a spot start against Las Vegas Western.
During a game last month against El Camino Real, Sementelli entered in the sixth inning with her team trailing by six runs. As the reliever finished her warm-up pitches, a female fan shouted encouragement from behind home plate: "Come on, girl!"
It was not one of Sementelli's better outings. Yet, even as the Conquistadores loaded the bases on a pair of walks and a single, Sementelli never appeared rattled. She continued to work at her usual breakneck pace, pausing only a few seconds between pitches.
When Tony Rizzo struck out chasing a pitch later in the inning, he was greeted by smirking teammates in the dugout.
"Everyone's going to give you a hard time," said El Camino Real first baseman Mitch Bluman, who struck out against Sementelli an inning later, "but it's all in good fun."
Only once has Sementelli's gender sparked a flap. When she was going for her 500th strikeout in the Sherman Oaks Little League, a local television crew came to document the feat.
Parents of the opposing team's players threatened to forfeit the game unless the TV crew left. It did, leaving Sementelli's father to record the milestone on his video camera.
A self-described baseball nut, Gary Sementelli always wanted a son. When Marti was born, he says he asked the doctor three times, "Are you sure it's a girl?" — as if he could somehow will a change.
Gary first put a small souvenir bat in his daughter's hands at age 2½, hoping to tire her out for a nap. But something about the game took, and she kept progressing from one league to the next, never once straying to softball.