Martina Hingis was surprised to hear about Kim Clijsters' planned comeback and believes the former world number one will find it difficult to cope with the physical demands of the sport.
Clijsters shocked the tennis world in September when she revealed she was training for a return to the professional game seven years after retiring for the second time.
She has already had to push back the date of her planned comeback from the start of next season because of a knee injury.
Hingis, who first played a match against Clijsters nearly 20 years ago, told the PA news agency: "Having had three children, 36 years old, I was surprised, yes.
"She might be playing great but it's also the recovery. For me, even playing doubles, that was the hardest part. The week in, week out. I already heard she's injured.
"I didn't want to say anything bad because it's great she wants this challenge but I think it's not easy. Being a mother now and having had a couple of comebacks, I don't think it's as easy as people sometimes think.
"I don't think the tennis will be a problem for her, she was number one in the world, she knows how to do things, but it's the body. And she's had injuries before.
"And it's not just the tennis, it's everything around. I was (at the WTA Finals) in Shenzhen and I talked with Aga Radwanska, who just retired at the end of last year, and she was also like, 'I kind of miss it sometimes when I'm watching, but you appreciate it even more because you're not ready to put all the work into it'.
"To play at Wimbledon on Centre Court, that's the easy part. Getting there is the hard part, how much work and effort it takes."
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Hingis' career also played one in three parts. The Swiss retired for the first time in 2003 having won five grand slam singles titles as a teenager, returned in 2006 and 2007 and then came back again in 2013 solely as a doubles player.
She was enormously successful, adding 10 more slam titles in women's doubles and mixed doubles having accepted that her body would not cope with the rigours of singles.
Hingis, who retired for the final time at the end of 2017, said: "When you were a winner, when you were winning grand slams, it's not just about going and winning a first round.
"For me, that was not enough. I'd rather be number one in doubles, the way I finished, but not just be 50-100 in singles. And I don't think Kim would want to do that either."
Retirement has brought marriage and motherhood, with Hingis, now 39, giving birth to daughter Lia in February.
"The first three months was very intense," she said. "It's quite a different chapter in my life. I cherish every minute of it but no one really prepares you for it. But you learn to trust your gut and try to do the best.
"You're never going to be perfect but the tennis life kind of helps to have a routine. I also was lucky enough to have the luxury that I can do the things that I want and I have the time for her."
There will be no third comeback but Hingis can often be found on a tennis court and will take part in the Champions Tennis event at the Royal Albert Hall later this month.
She said: "I played there once before and I'm really looking forward to it because it was a great event and a beautiful atmosphere. Not everybody can say, 'Hey, I've played in the Albert Hall', right?"
Hingis is also planning to play in legends events at the grand slams next year, bringing baby Lia along with her.
Hingis is the daughter of two tennis players and was coached by her mother, Melanie Molitor, who has also guided the career of current world number eight Belinda Bencic.
So could Lia follow in the family footsteps?
"I wouldn't say no if she would want to," said Hingis. "It's a long way to get there. She already has a couple of tennis rackets and she's already been on the tennis court with me.
"She's ridden my horses. We try to keep it interesting for her and she's very excited about the horses and animals and tennis. She wants to know and see everything."
:: Martina Hingis is taking part in Champions Tennis 2019 at the Royal Albert Hall from November 28–December 1. Tickets start at £18.50 and can be purchased at www.championstennis.co.uk/tickets2019