The departure of Serena Williams from tennis will leave a void in the sport that may never be completely filled, according to the coach who played a pivotal role in nurturing her talent more than 30 years ago.

The 40-year-old 23-time Grand Slam champion bowed out of the US Open on Friday in what is widely expected to be the final singles tournament of her 27-year professional career.

For Rick Macci, the Florida-based coaching guru who oversaw the development of Williams and sister Venus in the early 1990s, Williams leaves a legacy that will never be beaten.

In the end, Serena fell just short of matching Margaret Court's all-time mark of 24 Grand Slam titles.

But for Macci, the totality of his former protege's achievements, on and off the court, represents a more fundamental and lasting legacy than a mere tally of titles won.

"She's the greatest female tennis player ever to hold a racquet, and the greatest female athlete that we've seen," Macci told AFP.

"I tell other people she should go on Mount Rushmore not just because of the size, the speed, the quickness, the agility, the best serve ever, the ability to dominate you.

"No, she should be on the Mount Rushmore because when you played Serena she literally made everybody 'rush more'."

- Checking every box -

Macci believes Williams' array of technical talents and physical strengths, allied to her remarkable longevity and an indomitable spirit honed by her upbringing in Compton, made her unique.

"You're never going to see a player like Serena Williams again," said the 67-year-old.

"She checks every box - size, speed, quickness, strength, agility. She had muscles on muscles even at aged nine.

"Technically she hits the ball very clean. We moulded the best serve in the history of tennis. But at the end of the day it's that Compton street fight.

"Serena was like a pit bull. When she got a hold of you she wouldn't let go. And that's an amazing quality. She has no weaknesses."

In addition to Williams' 23 singles Grand Slams -- part of an overall haul of 73 titles -- she also added 14 doubles titles with sister Venus and four Olympic gold medals.

Macci, who has also coached world number ones Andy Roddick, Jennifer Capriati and Maria Sharapova in a Hall-of-Fame career, believes Williams could easily have won more titles.

"She didn't play all the Grand Slams. She was injured. She had a child. She could have had 30 Grand Slams.

"But it's not even about the numbers. They just kind of mesmerize you when you hear them all. She could do it all. She's the mentally strongest female athlete that we've ever seen."

- Doubles encore? -

Williams departs the stage at a time when there is no obvious successor in the women's game. Macci said the shock retirement of Australia's Ashleigh Barty, and the inconsistent form of Naomi Osaka, reflected a sport in a state of flux.

"There will be a little bit of a void," Macci said. "It will take time to create rivalries, and that's why women's tennis right now is very fluid.

"For someone to be the face and dominate like Serena -- you're never going to see that again.

"Forget the numbers, the numbers are off the charts. And that's one thing. But you're never going to see someone who has influenced so many people around the world."

Macci does believe, however, there is a strong chance Williams will continue to grace the doubles court alongside sister Venus.

"I could see her and Venus playing doubles two or three years," he said.

"You only cover half the court. They've both got good serves. Their return of serve is brutal. I could see them still playing doubles and mixed doubles. Nothing would surprise me.

"I don't think we've heard the last of either of the sisters."

A renaissance of the Williams sisters doubles partnership would provide a satisfying bookend to their careers for Macci, who first saw the siblings play on a rundown court in Compton 31 years ago.

"At age nine, Venus and Serena were skipping and holding hands," he said.

"And even now in their 40s they’re still skipping and holding hands. Two peas in a pod - like no other."