Serena Williams: How US Open victory of 1999 tells the story of what was to come

"I went and told my wife we had to have two more girls and make them tennis players," he recalled, 20 years later. "I was 37 and knew nothing about tennis but I thought we could teach them and they could win the US Open."

On 26 September 1981, Williams arrived - 15 months after her sister Venus. By the time the pair turned pro in the mid-1990s they were already making waves. 

"There was a rage inside these two little kids," says Rick Macci, the Florida-based coach who helped guide them through their developmental years. 

"They were just ready. I called it when they were nine, 10 years old. I said: 'They're gonna be world number one and they're going to transcend the sport.'"