Why you've never heard the voice of Mirka Federer, Roger Federer's wife.

Mirka Federer, the wife of tennis legend Roger Federer, is a familiar face court side.

While she’s a constant presence at Roger’s matches, a new biography about the Swiss player has explained why we never hear her speak to the media.

Simon Graf, a long-time sports editor in Switzerland, released a book on the 2-time Grand Slam winner in October and explained more in a January interview with tennisnet.com.

Graf said that Mirka was advised by her husband more than a decade ago not to speak to media in order to protect her privacy.

"He advised her over ten years ago. Roger likes to keep control, he does not want everyone in the team talking," Graf said in the interview (Google translated from Swiss German).

"Physio Daniel Troxler does not speak, [Coach] Ivan Ljubicic does not, Mirka does not speak either. Severin Lüthi [Federer's long-time head coach] speaks, I think that's good too. Because he is always underestimated. By now people know what he understands about tennis, that he is an important figure for Roger.

"Otherwise: Roger speaks a lot, so he makes up for everything. But he also does not want to reveal private details."

Even now, unlike other public figures, Mirka Federer doesn't have an Instagram or Twitter account - so doesn't communicate directly with the public.

Graf said Roger is very conscious in how he leads his career, which is why he has been so successful.

"He just wants to know very well what's going on... Federer is always very nice, but he is not one who always wants to please everyone. He made his way very straightforward and made tough decisions along the way."

Mirka and Roger Federer met at the Sydney Olympics in 2000.

The pair married in 2009 and she’s often seen in the player’s box with the couple’s four children, identical twin girls, Myla Rose and Charlene Riva, eight, and twin boys Leo and Lenny, four.

Born in Slovakia in 1978, Mirka emigrated to Switzerland when she was two years old. When she turned nine, her father took her to watch a tennis tournament in Germany where she met former number one Martina Navratilova, who suggested she try the sport. Later, Navratilova reportedly sent her her first racquet and arranged a tennis lesson for her.

Years of training and international tournaments followed before she competed in her first Grand Slam at the 1999 French Open, exiting the first round. In 2000, she played in the Australian Open, French Open and Wimbledon, as well as the Sydney Olympics.

She cracked the top 100, reaching a career-high WTA singles ranking of world No. 76 on the 10th September 2001. That year, she also made it to the third round of the US Open.

She retired early in 2002 due to a persistent foot injury.

Unable to play the game she loved, Mirka became involved in other ways, including becoming Federer's public relations manager, handling much of his media affairs.