When, in 2016, Kerber stunned Williams to win the Australian Open and beat Karolina Pliskova in the U.S. Open final, losing to Williams in the Wimbledon final in-between, it seemed she had marked herself out as true rival to the American.
Navratilova has been as impressed as anyone watching Williams's Wimbledon return, the American having been absent a year ago as she prepared to give birth to daughter Alexis Olympia.
Williams was bedridden for six weeks after giving birth to daughter Alexis Olympia - she had an emergency caesarean section, which is major abdominal surgery, followed by a series of complications including the wound "popping open" because of a pulmonary embolism, and a swelling of clotted blood in her abdomen.
"I think it's incredible that she's even had time and made the effort [to get back to that level]", she said. "Physically I was surprised just how much of my core strength I had lost", she said. "I had a really tough delivery and I had to have multiple surgeries and almost didn't make it when I gave birth", the champion said.
She is by far the most unsafe player the American has faced so far this fortnight and is on something of a comeback trail herself after a slump in 2017. "Because of all the blood issues I have, I was really touch-and-go for a minute".
"If I want to play on that level, I have to reduce my unforced errors".
After hitting five aces with a serve that reached 119 miles per hour, delivering 16 winners to only seven unforced errors, and covering the court so well with speed and effort, Williams will face another German, 11th-seeded Angelique Kerber, on Saturday.
"I honestly felt I would have done better in some of the earlier tournaments. So do I", Williams said.
"A lot of people were saying, 'Oh, she should be in the final, '" the 36-year-old Williams said.
She'll play Angelique Kerber for Grand Slam No. 24.
Those two contests suggested Kerber could be the rival Williams often has lacked in the latter half of her glittering career.
Wimbledon is just Serena's fourth tournament since returning to the tour in March. In a catsuit that was as much a health precaution as a style statement, it was evident she wasn't a 100% physically, yet she persevered with her mix of power and resolve, before a pectoral injury forced her to pull out in the fourth round.
Next up is a familiar opponent in Serena Williams, a woman who has every shot in the book and the experience to also adapt her game to her opponent.
"She's playing so well", Williams said of Kerber.
In a sense, Williams has been writing this narrative - rising up, overcoming, doing the impossible - since her father, Richard, took her and sister Venus to a dilapidated tennis court in Compton, Calif., put rackets in their hands and coached them, along with their mother, into champions.
The American's fiercely competitive streak fuelled her rise from the bullet-riddled courts of Compton in south-central Los Angeles, and she remains as driven as ever. There was a time I could barely walk to my mailbox. She's here to win. To see her back, it's great.
Meghan Markle has been pictured at Wimbledon on several occasions in the past, including in the summer of 2016, the summer in which she is thought to have met Prince Harry for the first time.
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