World number one Williams is already plotting a return to tennis next year after announcing she is pregnant, saying she "can't wait" to introduce her first child to the players' box.
She returned to number one in the women's rankings on Monday even though she has not played professionally since cruising to the Australian Open title nearly three months ago.
Ennis-Hill took time out of athletics in 2014 and 2015 after having her first son Reggie but returned to win silver at the Rio Olympics last year.
The 31-year-old is now pregnant with her second child and the 2012 Olympic heptathlon champion believes Williams will be able to juggle her responsibilities.
"It's very individual to everyone and to what sport you do," said Ennis-Hill, speaking at the launch of running festival VitalityMove.
"Tennis is very different to athletics, it's an incredible thing to do. It's probably one of the hardest things you will face in life as a women, having a child and coming back to sport.
"I'm sure whatever she decides she'll do a great job at it because she's a fantastic athlete.
"Your life changes so much and you do change as a person massively. You can have benefits, when I came back and did the 800 metres I found that was a lot easier than previously - but then the other six events were harder.
"It's a balancing act but you can take the good things from being an athlete to help you come back."
A by-product of 35-year-old Williams announcing she was 20 weeks pregnant last week was that she claimed an Open-era record 23rd grand slam title, without dropping a set, in the early stages of gestation.
Williams, who became engaged to Reddit co-founder Alexis Ohanian in December, will miss the rest of the year but is relishing her comeback in 2018 with a newborn baby in tow.
Alongside a selfie, Williams wrote on Instagram: "My Dearest Baby, You gave me the strength I didn't know I had. You taught me the true meaning of serenity and peace. I can't wait to meet you. I can't wait for you to join the players box next year.
"But most importantly, I am so happy to share being number one in the world with you.... once again today. On @alexisohanian bday. from the world's oldest number one to the world's youngest number one. -Your Mommy."
