The correct answer, in fact, was 19 years and two days, and Williams had rightly remembered that her centre court debut had not come against her sister Venus, as advised, but in the first round of her first-ever grand slam against family foe Irina Spirlea, back in 1998.The 16-year-old Serena won it, 6-7 (5-7), 6-3, 6-1, and has pretty much been winning ever since.

Each Williams has contested 17 Australian Opens; together they have featured in the last 16 of the same slam an extraordinary 35 times, including this one. Venus meets Aussie-slaying German qualifier Mona Barthel on Sunday, while Serena's 11th quarter-final appearance at Melbourne Park rests with victory on Monday against Czech 16th seed Barbora Strycova.

Happily, the siblings are in separate halves of the draw, unlike that first occasion, when the prodigies met in the second round. The first of the 27 times they have played in official tour-level matches was when 17-year-old Venus won in straight sets. It was, yes, exactly 19 years ago.

"I remember the draw came out, and I had to play her in the second round. I remember, I think, we had a tough first set, and then she really went through in the second. Then she went on, I think, to the quarter-finals. She had a really good tournament that year.

"It was a great time. We really had so much fun playing. It's really exciting, looking back, and looking at those moments. You don't really get those moments back, but you can remember them so well. It's so fun. It was the first of so many."

Gibbs was a lesser-known, ranked and credentialled opponent than Williams' first two, Belinda Bencic and Lucie Safarova, but the scoreline was not too dissimilar, and Serena's satisfaction levels high as the second seed but title favourite edges towards grand slam singles crown No.23.

"I was so pumped up going against my first two opponents, but I think that helped me out today," Williams said. "[Gibbs] started out really, really well. Started with a lot of energy, and then I felt like at the same time I really needed that."

For Gibbs, playing her idol brought its own challenges. And rewards. "She was really nice at the net. She said [she'd seen] great improvements since the last time she played me, which is obviously a nice comment from one of my heroes, so definitely taking that to heart and not being too critical of how I played today. Obviously, all credit in the world to her for being the incredible tennis player that she is.

"She makes the court feel very very small. I think that's the toughest thing about being across the net from her. I think she's in a league of her own. Especially for me, the Williams sisters are who I grew up watching on TV. I think she turned pro when I was three or something, so I have literally been watching her for the entirety of my tennis life. I think that does play into how you feel on the court. It becomes more than a tennis match."