In fact, their father was planning their meteoric tennis success even before they were born.
Raised in Compton, Los Angeles, with three other sisters, the pair were competing in tournaments at the age of five and turned pro in 1995. After years of playing tennis in private competitions, and coming to the attention of coaches at national level, Venus pipped her sister to a first major title, winning the US open in 1999.
However, Serena was crowned World No.1 in 2002 – and for the moment, after an incredible 2012, she is still the more successful player.
At the Summer Games in London the 30-year-old overcame all before her in the Singles tournament, losing just 17 games in six matches and only one in the final to Maria Sharapova of Russia.
And she achieved what Roger Federer could not in the men’s game, adding the Olympic champion title to her Wimbledon crown and completing her career Golden Grand Slam.
But that was not the end of Serena’s historic summer, as she teamed up with Venus to win a third women’s Doubles title – giving each sibling four tennis gold medals, more than anyone else in history.
After breaking into the top 10 in 1999 – when Serena also won her first professional singles title, the Open de France in Paris – both sisters suffered injury and experienced dips in form over the next decade.
Serena missed out on representing Team USA at the Athens Games in 2004, but amassed an incredible haul of Grand Slam titles, winning the French Open, five Australian titles, four Wimbledon Championships, three US Open tournaments and 11 doubles crowns.
Despite being regarded as one of the greatest tennis players of all time, she slipped in and out of the top 10 rankings between 2000 and 2007, only regaining her No1 status two years later.
But with her aggressive forehand and unstoppable 128mph serve – only Venus hits harder – it would have been unwise to write off the younger Williams sister.
Coming into London 2012 she was the present holder of all four Grand Slam singles titles, only the fifth woman to achieve such a feat.
After steamrollering Sharapova 6-0, 6-1 in the singles final, the sisters teamed up to see off Czech duo Andrea Hlavackova and Lucie Hradecka 6-4 6-4.
The sisters set out their stall early on Centre Court at Wimbledon, breaking serve in the very first game.
Their powerhouse performance dominated the Olympic tournament. And after she won the US Open weeks later, Serena’s career tally came to an astonishing 30 Grand Slam titles and four Olympic medals.
Serena’s Wimbledon gold medal made her only the second player after Steffi Graf to win a career Golden Slam, and she was the first player in history, male or female, to achieved the feat in both singles and doubles.
Later she said the tournament was the best she has played from start to finish.
She said: ‘I was so focused here. I remember I was serving and I was thinking: ‘Serena, this is your best chance to win a gold medal. You’re at Wimbledon, you’re on grass, you play great on grass, pull it together, just win this.’ And that’s what I thought about.’
After the doubles final Venus said: ‘For us it’s so exciting winning the gold together. For me it was amazing to watch Serena from the stands and see her complete the Golden Slam. So it’s been amazing for us.
‘We’ve been winning this title since 2000, but it’s easier said than done. We come in as the favourites. But it's not a given. We fought hard. Our opponents played well. They're very talented. We're glad to keep the medal for us. It was great.’
Now into her thirties, Serena shows no sign of letting up – stating the day after winning singles gold that London would not be her Olympic swansong.
She said of Rio 2016: ‘I plan on being there unless an act of god doesn't allow me to.’