"To my friend, the great champion" - Billie Jean King wishes Steffi Graf a happy 54th birthday

Women's tennis legend Billie Jean King wished fellow tennis great Steffi Graf on her birthday and recalled all the great things the German icon achieved in her career.

Graf turned 54 on June 14 and was wished by many in the tennis world. Among them was trailblazing tennis star King, who took to social media to send a very special message to Graf.

The 79-year-old mentioned some of the milestones from Graf's glorious career, like 22 Grand Slam titles, 377 weeks as the World No. 1 women's tennis player, and an Olympic gold medalist in Seoul in 1988.

"22 Grand Slam titles, World No. 1 for 377 weeks, and Olympic gold medalist. Happy birthday to my friend, the great champion, Stefanie Graf!" King tweeted.

Graf is one of the living legends of tennis. Her professional career spanned from 1982 to 1999, during which she became the only tennis player, male or female, to win each Grand Slam tournament at least four times. In 1988, she also became the only player in the history of the sport to achieve the Golden Slam as she won all four Grand Slam titles, as well as the Olympic gold medal in the same calendar year.

Graf ended her career with 107 singles titles, the third most among the women. She was also given the honor of the greatest female tennis player of the 20th century by the Associated Press in 1999.

Where is Steffi Graf now?

Steffi Graf and Andre Agassi

After retiring from tennis in 1999, Graf married American tennis star Andre Agassi on October 22, 2001. The two began dating during the 1999 French Open, which they both won. Since then, Graf has been leading a very private life in Summerlin, Las Vegas, where she lives with Agassi and their two children, Jaden and Jaz Elle.

Graf has also played many exhibition matches after her retirement, most famously facing old rival Martina Navratilova in 2008 at an event in Tokyo. The German managed to get the better of Navratilova.

She also teamed up with Agassi to play a mixed doubles match at the 2009 Wimbledon Championships to inaugurate the newly constructed roof over Centre Court. However, the couple lost to Kim Clijsters and Tim Henman.

Steffi Graf has also been involved in charitable ventures during and after her career. She founded 'Children for Tomorrow' in 1998 to help kids and adolescents adversely impacted by war and organized violence. The charity is located in Hamburg, and Graf is extremely engaged in its operations.