"I saw her in the locker room and she was another black girl so I was like, 'Hey'," Williams said, breaking into laughter. "That's when I first noticed her. I said, 'What up, girl?' "
A lot has changed since then and tomorrow, the young girl she befriended will be facing Serena across the net, battling it out for a place in the semi-finals of the Australian Open.
The 19-year-old Stephens followed up her win over Britain's Laura Robson by ousting Serbia's Bojana Jovanovski 6-1, 3-6, 7-5 to reach her first grand slam quarter-final.
Williams, seeded No.3, crushed Maria Kirilenko 6-2, 6-0 to storm into the last eight in Melbourne.
In the last couple of years, Williams has taken Stephens under her wing and the young American pushed her pretty hard when they met in Brisbane a fortnight ago, prompting a bold prediction from the former world No.1.
"Like I said after the match, she can be the greatest player," Williams repeated yesterday. I feel like, for me, it will be another good match and a good opportunity for both of us. Definitely one of us will be in the semi-finals, which I think is awesome. So, I think it's going to be good."
Inevitably, there have been comparisons between Williams, who has won 15 grand slam titles and is bidding for her third straight slam win and Stephens, whose court coverage is reminiscent of the world No.3.
Williams said she was not quite a mentor to Stephens and would not go easy on her just because the pair are friends.
"It's hard to be a real mentor when you're still in competition," she said. "So I think it will be a little bit of everything."
Yesterday, Williams showed no sign of discomfort to her right ankle after rolling it in her opening round match, crushing her Russian opponent with minimum fuss.However, Stephens is determined not to be overawed and believes that the experience of playing her fellow American in Brisbane will help when they play on Rod Laver Arena tomorrow.
"That was two weeks ago now," she said. "I've played seven or eight other matches, so I'm kind of past what happened then but I think I'll definitely treat it as another match. You just go out and do your best."
"She's obviously one of the greatest players to play the game. Without all that, it's still a tennis match. You have to go out and play your game, no matter what. Without the titles, with the titles, it's still a tennis match."
Top seed Victoria Azarenka produced her best performance of the fortnight as she hammered Russian Elena Vesnina 6-1, 6-1.
She will now play Svetlana Kuznetsova, the former US Open and French Open champion who is unseeded this year but showed she's coming back to form by beating No.10 seed Caroline Wozniacki 6-2, 2-6, 7-5.