NEW YORK • Earlier this month, Serena Williams spent a week visiting her trainer Mackie Shilstone in New Orleans. She went to his home, rode a bicycle around the city and worked in his gym. Never did she reveal she was four months pregnant.

He learnt that secret on Wednesday, just like everyone else, when a photo of the No. 1 women's tennis player with a baby bump appeared on Snapchat with the words: "20 weeks."

Looking back at Williams' visit, Shilstone now can see the signs. She did not talk a lot about upcoming tournaments, or how her January Australian Open win - presumably while pregnant - put her at 23 Grand Slam singles titles, just one from the all-time record.

Instead, she headed for a vacation with her fiancee Alexis Ohanian.

But, even as Shilstone processed the news of her pregnancy, he was thinking about the challenge of meeting her goal of returning to tennis next year.

"Without a doubt I think she will come back and be even stronger - because she will be playing for two," he said. "If she sets foot on the court she will come to play. I hate using the word "star" but Serena is a star in the true sense. She is just unique."

Several athletes have had children and come back as effective or even better than before.

Belgium's Kim Clijsters had a baby in 2008 and then won the US Open in 2009 and 2011, as well as the Australian Open in 2011.

Margaret Court Smith, whose Grand Slam record Williams is chasing, won three of her 24 championships after having a child in 1972 at age 29.

Britain's Jessica Ennis-Hill won gold in the heptathlon in the 2015 World Championships at 29, a year after her son Reggie was born, adding a silver Olympic medal a year later in Rio.

Liz McColgan won the 10,000m at the 1990 World Championships at 26, a year after having a child, while Paula Radcliffe won the 2007 New York Marathon at 33, only 10 months after giving birth.

But the return is not easy. Many athletes feel like they are starting over and find even the simplest exercise tough. The come back, they learn, is agonising.

"Your body just feels so different," Clijsters said during her 2009 US Open run describing her first tennis sessions after childbirth. "I had a good feeling when the ball was coming towards me, but just moving was absolutely terrible. I felt like an elephant sometimes, just trying to move."

Williams has a goal of returning next year, roughly matching Clijsters' path from nearly a decade before.

The difference between her and Clijsters, however, is that the Belgian was 26 when she returned to tennis. Williams will be 36 in September - an age at which many top players have already retired.

Not only will she be trying to defy time, she essentially will be learning to play all over again.

Not that Shilstone believes this will be an impediment. "What I've learnt over the last several years is that Serena has some of the best discipline that I've seen," he said.

Since she has still not told him about her pregnancy, he did not want to detail a definitive plan for her return to tennis.

But he has run wellness programmes at a New Orleans hospital for new mothers, and, assuming that Williams will take about three months after giving birth to bond with her baby, he described a general approach to getting her into peak condition by early next year.

He said he will first work to "reestablish (Williams') female physiology", reversing the cardiovascular and metabolic changes that will have taken place in her body before and during birth.

Next he will "re-establish baseline fitness", building back the strength and endurance she will have lost.

Then, once she is in shape again, he will work on her fine motor skills - the eye-hand coordination that allows her to place a ball almost anywhere she wants.

Shilstone, who started working with Williams in 2008, has pushed her through other comebacks in the past, the most significant being in 2010 after she cut herself on a piece of glass and developed blood clots in her legs and eventually her lungs, nearly killing her.

She missed nearly a year of tennis and lost a great deal of strength and stamina. He worked her hard in the months leading up to her 2011 comeback, using tactics devised for US Navy Seals.

When challenged, Serena always responds, says Shilstone. "I know how she can come back," he said. "I know what she can do."

British long distance runner Jo Pavey, who had her first child in 2009 at the same age as Williams will, said she could not wait to return to running, starting just three weeks later.

Clijsters found yoga helped her body recover as she came back to tennis. But what frustrated her the most was how she could not do things that once seemed natural.

As Shilstone suggested in his basic recovery plan for Williams, the Belgian former world No. 1 was startled at how much core strength she lost. Her lower back and stomach had always felt muscular. But, after a baby, they had grown weak.

"So you have to reteach all those muscles to contract when they're supposed to, especially with each shot that you hit," she said during that first US Open back. "I mean everything just has to get reminded of: 'OK, this muscle has to move at that time'."

Shilstone thought about this as he remembered something that happened during Williams' visit earlier in the month.

They were riding bicycles on a pothole-filled road and at one point Williams wobbled, nearly falling. Oblivious to her pregnancy, Shilstone laughed. "Are you an athlete?" he called to her, jokingly. "Well, maybe you know I can play tennis," she shot back.

Somehow, he figures that even after she has a baby at 36 she will still know how to do that.

THE GUARDIAN