Her best Grand Slam singles tournament was when she reached the 1970 French Open final, losing to Margaret Court in straight sets.

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.

[1] At the 1968 Olympic Games in Mexico City when tennis was a demonstration sport, Masthoff won the singles and doubles (with Edda Buding) gold medals and the silver medal in mixed doubles (with Jürgen Faßbender).

[2] In 1970, she received the Silbernes Lorbeerblatt (Silver Laurel Leaf), the highest sports award in Germany.