The correct answer is Martina Hingis. She became the youngest number 1 tennis player in the world at the age of 15 years and 9 months.
Steffi Graf became the number 1 tennis player in the world at the age of 17 years and 3 months.
Monica Seles became the number 1 tennis player in the world at the age of 17 years and 2 months.
Chris Evert became the number 1 tennis player in the world at the age of 20 years and 2 months.
Martina Hingis was born on September 30, 1980, in Košice, Czechoslovakia. She began playing tennis at the age of five and turned professional in 1994. In 1995, she became the youngest player to win a Grand Slam singles title at the age of 15 years and 9 months. She also became the youngest player to reach the number 1 ranking in the world at the same age. Hingis retired from professional tennis in 2002, but returned to the tour in 2006. She retired for good in 2007.