Happy birthday, Tracy Austin!

Did you know Tracy Austin's sister-in-law is fitness guru Denise Austin? Focus on Sport/Getty Images

Happy birthday, Tracy Austin! The two-time US Open champion and the former No. 1 tennis player in the world turns 51 today. After turning pro in 1978, Austin became the youngest champion at the 1979 US Open at the age of 16 years, 8 months and 28 days. She stunned the tennis world with victories over Martina Navratilova in the semifinal and Chris Evert in the final. The following year she won her lone Grand Slam mixed doubles title at Wimbledon with her brother John. Despite chronic injuries, she was healthy enough to win the 1981 US Open title, but she was sidelined shortly after for long stretches due to back pain and recurring sciatica. She attempted multiple comebacks, but officially retired in June of 1994. She became the youngest person to be inducted into the International Tennis Hall of Fame in 1992 at the age of 29. Since hanging up her racket, Austin has been a tennis commentator for various networks and lives in Los Angeles with her husband and three sons.