Growing up in Bethlehem, PA, Thesieres played intercollegiate tennis and badminton at Ursinus College. A protégé of Norm Bramall, Thesieres learned squash at Cynwyd Club starting in 1962. She soon was a top-three ranked player and was a key member of the Philadelphia teams that dominated the Howe Cup in the 1960s. After losing in the finals of the National Singles for two straight years, the left-hander won the title in 1971, topping Goldie Edwards in the final. She soon switched to doubles. With her precise reverse corner and powerful strokes, Thesieres became one of the most feared left-wallers in history, taking nearly every doubles tournament title in the game. With partners Joyce Davenport and Jane Austin Stauffer, she captured the National Doubles seven times in three different decades: 1969, 1970, 1975, 1979, 1980, 1981 and 1982. She won the National Mixed in 1979 with Gil Mateer. She captured the women’s 40+ doubles title with Irma Brogan in 1980, 1981 and 1982, becoming one of just three players to win the open and 40+ draw in doubles in the same year and the only one to do it three times. Thesieres was also a longtime leader in the game, chairing tournaments and serving as the president of the Philadelphia women’s association in 1971-72.