Australian great Margaret Court won 24 grand slam singles titles, one ahead of American Williams, who confirmed she was pregnant on Wednesday and will sit out the rest of the season.
Court, 74, who straddled the sport's amateur and professional eras, was also the first mother to win a major as a professional. She took three of those titles in 1973, the year after giving birth to her first child Daniel.
But she rejoined the tour at 31 while Williams would be 36 if she came back as planned in 2018, a difference Court thought might play a role.
"Coming back after Daniel I had one of my best years ...(but) I was quite a few years younger (than Serena)," Court told Reuters in an interview on Thursday.
"And I think you’ve got to know physically what you can do and how you can do it. You put the baby first and that to me was everything at the time. But we’re all different."
The only other mothers to win slams in the Open era are Court's compatriot Evonne Goolagong and Belgian Kim Clijsters who were both 26 when they came back.
"I don't know (if Serena can win another) ... It will depend whether she still feels like doing it. It’ll depend on her," Court said. "It depends who’s coming through, too."
'THERE'S NOT THE DEPTH'
On that point, Court felt women's tennis could still be at Williams's mercy, given no players had emerged to provide a sustained challenge to the American's dominance.
"There’s not the depth in the women’s game like there is in the men’s. It’s not good for women’s tennis which is a shame," she said.
"It’s like they have one win and they think 'well, that’s it.' They’re made. It’s a bit sad in many ways.
"It was more open in my years with Billie Jean King, (Martina) Navratilova and (Chris) Evert-Lloyd."
Williams's "20 weeks" caption in a social media post on Wednesday suggested she was two months' pregnant when she won her 23rd grand slam at this year's Australian Open.
Court was also well into her term with Daniel when she lost the 1971 Wimbledon final to Goolagong. But she had no idea.
"Our first child was with me on the centre court. My coordination, my timing was all out and I thought: ‘what’s wrong with me?’
"Balls were dropping in and I was letting them go. Nobody knew. I went to the doctor and then I found out."
Court promptly stopped playing and was initially unsure if she would return.
But after giving birth, the former world number one's mind was made up and she began jogging around a park near her house with a pram to get fit.
Court and her husband Barry juggled baby Daniel alone, and he was often wailing in the grandstands during matches.
That situates her a world away from the comeback that, in the age of social media and multi-million dollar celebrity endorsements, would await Williams as a playing mother.
"It was nothing like they have today. They’ve got the money to do it," Court said of modern players with children.
"They could have their own planes. But we coped very well."
Marketing experts have said Williams's pregnancy will boost her commercial appeal and open up lucrative avenues into maternal-wear and baby clothes.
Court chuckled at the idea. "We didn't have that," she said.
"I remember I’d signed a contract with Yamaha and it was on all the billboards in America and they had to take them down because I couldn’t play (due to pregnancy). Different times.
"But players are very blessed in these times and they should appreciate what’s been built before them. I don’t think they should ever forget that."
(The story fixes typo in first paragraph)
(editing by John Stonestreet)
