Both sisters turned pro at the age of 14. The younger sister Serena (left) born in 1981, won her first Grand Slam, the 1999 US Open, just days before her 18th birthday. However, Venus, born in 1980, was the more consistent back then, winning both Wimbledon and the US Open in 2000 and 2001. She became the world No. 1 for the first time at the start of 2002.

The two siblings met in a Grand Slam final for the first time at Flushing Meadows in 2001. Venus (lef) won 6-2, 6-4, but this would be her last success for quite some time. Between May 2002 and June 2003, Serena beat Venus in the finals of the French Open, Wimbledon, the US Open, the Australian Open and Wimbledon again. This made her the new world No. 1.

Their on-court rivalry in the singles game has had no negative effect on the sisters' relationship. When they can fit it in, they play as a doubles tandem, and have won 24 tournaments, including 14 Grand Slams. Their last title was at Wimbledon in 2016.

The sisters have also won Olympic gold in the women's doubles tournament in Sydney in 2000, Beijing in 2008 and London in 2012. Each has also won a gold medal in the singles. Their experience in doubles at the Rio Games last year was not so pleasant, though, as they were eliminated in the first round.

The driving force behind the sisters' success is their father Richard. He coached them from when they were little, forming the world-class tennis players they would later become. Richard Williams, who has 10 children from three wives, raised Venus and Serena as Jehovah's Witnesses. It is said that this meant that they were not allowed to have any friends beyond their own family.

The sisters' mother, Oracene Price, accompanies her daughters ariund the globe. She and Richard Williams divorced in 2002. In the early years, rumors were rife that the family decided which sister should win any direct match between the two. Following their parents advice, Venus and Serena also have a portfolio that includes a fashion label and co-ownership of NFL franchise Miami Dolphins.

Early on, Venus was the better player but has been in her sister's shadow recently. In 2011 she was diagnosed with Sjörgen syndrome, an autoimmune disease that causes muscle pain and fatigue. Venus plummeted in the world rankings and has only returned to full fitness since switching to a vegan diet.

Just as her sister slipped down the pecking order, Serena began to dominate women's tennis. She took top spot in the world rankings in February 2013 and stayed there for three-and-a-half years. She now has 22 Grand Slam titles to her name, as many as Steffi Graf, with whom she shares the record for weeks spent as world number one (186).

Serena Williams had to concede number one status to Angelique Kerber (r.) recently. Victory in Melbourne, however, would see her return to the top. And with 23 Grand Slams, she would become the world-record holder. Will her sister stand in her way?