In Thursday morning's third-round match at the Sleepy Hollow Country Club, Martinez, who will start at Auburn next month, also surprised Angela Jermain, of Columbus, Ga., 1 up.
Martinez, the youngest player remaining in the field, is playing in her first-ever USGA tournament. Her father, Julio, is her caddy.
'He gives me confidence, which is very important', said the teen-ager. 'I have to play, but, his role is important, too. When it comes to selecting a club, however, I make the decision.
'I consider it an honor to be playing here and I'm happy and excited. I've worked hard to get this far and I'll continue tomorrow. Hopefully, that will get me to the finals.'
Jermain was a member of the Curtis Cup team that defeated Great Britain and Ireland earlier this month.
Nirapathpongporn, entering her junior year at Duke, had eliminated Carol Semple Thompson, another Curtis Cup player, 5 and 3.
At 53, Thompson is the oldest player in the tournament. She won the title in 1973, and also has eight other U.S. Golf Association championships, including the last three in the U.S. Women's Senior Amateur.
As a result of her victories, Martinez opposes Brandi Jackson of Belton, S.C., who defeated Vikki Laing of Scotland, and Mollie Frankhauser of Columbus, Ohio, on the 20th hole.
Also a semifinalist is Becky Lucidi of Paway, Calif., who opposes Australian Lindsay Wright.
Lucidi, who plays at Southern Cal, started the day with a 2-up victory over Elizabeth Janangelo of West Hartford, Conn., then topped Curtis Cupper Emily Bastel of Upper Sandusky, Ohio, 2 and 1.
Wright, entering her senior year at Pepperdine, beat Erica Blasberg of Corona, Calif., 5 and 3, and then Walailak Satarak of Thailand, 4 and 3.