Chris Evert became the youngest tennis player to reach the semifinals of the U.S. championship in 1971. She reigned for 7 years as the #1 player in the world.
Jim Courier : Tennis Star
“Jim” Courier, Jr. (born August 17, 1970) is a former World No. 1 professional tennis player from the United States. During his career, he won four Grand Slam singles titles, two at the French Open and two at the Australian Open. He holds the record for being the youngest man to have reached the finals of all four Grand Slam singles tournaments, at the age of 22 years and 11 months.