She won the German Open three consecutive years from 1972 through 1974, beating Martina Navratilova in the 1974 final in three sets. Masthoff was the runner-up at that tournament in 1971, losing to Billie Jean King.
Between 1972 and 1974, she won the German Open title three times, defeating Martina Navratilova in the 1974 final after being runner-up to Billie Jean King in 1971.